Tainted Lazarus is a character added in The Binding of Isaac: Repentance. He is the alternate version of Lazarus, playable in two different states: Tainted Lazarus and Dead Tainted Lazarus. Tainted Lazarus has ragged hair and is covered in dirt while Dead Tainted Lazarus has pale white skin, glassy eyes, and is covered in blood. Horizontal scars are noticeable on the wrists of both forms of Tainted Lazarus. Tainted Lazarus is unlocked by reaching Home and using either the Red Key or the Cracked Key to access a special room in one of the walls while playing as Lazarus.
Tainted Lazarus starts with 3 red hearts, whereas Dead Tainted Lazarus starts with 2 soul hearts. Tainted Lazarus begins with fairly low range while Dead Tainted Lazarus has a 1.5x damage multiplier, slightly lower tears and speed, and -2 luck. Each character has separate items, attributes, trinkets, and consumables, similar to Jacob and Esau except each character is only played individually. Whenever a room is cleared, including each wave of multi-wave rooms like in Greed Mode and Challenge Rooms, Tainted Lazarus swaps between the two forms, essentially requiring the player to play each character on every other room. Tainted Lazarus starts with Flip in his consumable slot, which can be used to swap state immediately.
Flipping between forms will effectively act as a room transition for certain items; in other words, flipping back in the same room will reset those items as if a new room had been entered.
If the console command "restart 38" is put into the console, it is possible to start a run as Dead Tainted Lazarus without having to clear a room, however, you will not be able to change into Tainted Lazarus, whether that is from using Flip or from clearing a room.
Items that grant extra lives (Dead Cat, 1up!, Lazarus' Rags, etc.) only affect the one who picks them up. If Dead Lazarus collects an extra life item, it can only activate when Dead Lazarus is the active character, and vice versa.
If one form of Tainted Lazarus is reduced to zero hearts while being in the transitioning phase of Flip, he will remain alive as the other form but die as soon as he switches back. There is nothing the other form can do to prevent that death.
Quest items are counted as both characters having them.
The Polaroid and The Negative work on both Living Tainted Lazarus and Dead Tainted Lazarus, regardless of which picked them up.
Key Piece 1 and Key Piece 2 will grant their Angel Room bonus to both characters, and will combine to a whole key even if the pieces are not on the same character.
Only the character that picks up an activated item first gets the progress for related transformations, so the player needs to be aware of what character needs the transformation progress.
Tainted Lazarus takes a lot of micromanagement compared to most other characters. It can be difficult to accomplish simple tasks due to the forced switching between forms and Flip's long charge time. Try to stay ahead of this, and pay close attention to what form is active and when they will shift. Something such as collecting an item from a Treasure Room or Curse Room can potentially go wrong if enemies happen to be inside, as when they are defeated, Tainted Lazarus will switch form. This can lead to an undesired item on one form, or a forced use of Flip.
If an Angel Room has an item you would prefer on one form over the other, and Flip is not currently charged, using a bomb on the Angel Statue and fighting the Angel that spawns will make Tainted Lazarus flip upon the fight's completion.
Challenge Rooms and Boss Rush will flip Tainted Lazarus every wave.
Try your best to balance the forms' stats in a way that prevents runs from feeling "lopsided". An easy trap to end up in is one form feeling unstoppable, but needing to struggle through every other room with the other. Without use of Esau Jr., this is not a very sustainable strategy in later floors.
Try to boost Living Tainted Lazarus's range and damage, Dead Tainted Lazarus already has a large damage multiplier, so its less necessary to take large damage increasing items in this form.
Avoid high charge Active Items, as Items will only charge upon room completion for Tainted Lazarus's currently active form. This effectively makes any active item have double its charge time for Tainted Lazarus.
The player may desire to forgo Active Items entirely, as Lil Batteries and Battery Beggars will prioritize charging them over Flip, which may prove detrimental.
Tainted Lazarus's greatest strength lies in his precise control over his health bars. Using Flip strategically during boss fights is a key to victory. For example:
If one form runs out of Soul Hearts, but the other still has some, using Flip can help prevent loss of Devil Room or Angel Room chance.
Despite only being at half of his firepower at a time, Tainted Lazarus can utilize his potentially massive health pool to clutch out unlikely victories against late game bosses. If damaged down to lethal health, using Flip can extend the fight dramatically. This can be further improved by having one or both forms have healing options such as XIX - The Sun, Full Health Pills, or an Active Item like Guppy's Paw.
Due to this strength, it may be advised to try for Angel Rooms over Devil Rooms, as many Angel Room items add health rather than take it away.
If one form of Tainted Lazarus has flight and a room is cleared while above usually inaccessible land, the player will need to exit to the menu and continue the run to reposition the flightless form at a proper exit.
Greed Mode, and especially Greedier Mode, can be very difficult for Tainted Lazarus to the point that it is arguably one of the most difficult challenges in the game, for several reasons:
Each completed wave swaps from one form of Tainted Lazarus to the other, which locks him in the flip animation for about a second. He cannot fire tears while in this animation. Considering that waves in the later floors of Greed Mode can last 3 seconds or shorter, this leaves Tainted Lazarus unable to attack enemies for a significant amount of time.
Additionally, temporary buffs that are usually powerful in Greed Mode, such as The Book of Belial or XI - Strength, will disappear when you switch from one character to the other, meaning you can only use them for a single wave.
It may be best to favor one form of Tainted Lazarus over the other, as only one form has to fight Ultra Greed and Ultra Greedier. Defeating Ultra Greed does not cause Tainted Lazarus to flip characters. The room before Ultra Greed also doesn't cause the forms to flip.
The floor generation depends only on the character entering the trapdoor leading to this floor. Any aspect of the other character does not influence the floor generation.
As such, items that affect floor generation (for example Fragmented Card) work only when the trapdoor is used by the character holding the item.
The Planetarium chance is NOT shared between both characters as well. Therefore, the Planetarium chance for one form would increase if the treasure room was not entered by that form, and it is possible to see a second Planetarium without the help of Telescope Lens.
Steam Sale is not shared between the two, consider giving it to the least powerful one, especially in Greed Mode.
It is confirmed that Tainted Lazarus is in a Work In Progress (W.I.P) state, he and Tainted Apollyon were not fully developed due to release dates, they will be fully developed in further updates and. 
During Dogma + The Beast fight, flips happen between each boss. For example, if entering Home with Live Lazarus, Dogma will be fought with Live Lazarus, then Dead Lazarus will fight Ultra Famine, Live Lazarus will fight Ultra Pestilence and so on. Then, Dead Lazarus will fight the Beast. Flip does not activate between the different phases of the bosses.
Eucharist is not shared between the two - the form with Eucharist must be the one to defeat the boss to spawn the Angel Door.
Kidney Stone: After switching in form with Kidney Stone, the effect of item will activate immediately.
The Stairway: If you go down a floor while on the character not possessing the item, the Stairway will not spawn on that floor.
Urn of Souls: Neither character can collect souls when they are not active. This means that as soon as the room becomes cleared, all souls still in the air are lost, even after you Flip.
Lost Soul: If one character has lost soul and it dies, it will respawn after that character is active again. Going to the next floor with the character that doesn't have lost soul will still grant its rewards once the character is active again.
TMTRAINER will only generate glitched items if the form holding it is the one to see the item first. This can allow the player to get normal, unglitched items onto the form holding TMTRAINER, and glitched items onto the form without it if desired.
Notably, this allows for Tainted Lazarus to obtain quest items such as The Polaroid or The Negative despite holding TMTRAINER. To do this, the form WITH TMTRAINER must be the one to fight Mom, due to the Polaroid and Negative spawning slightly after Tainted Lazarus flips forms.
Blood Puppy Will rarely rage as it needs to get all 15 kills in the same room.
There's Options will only take effect if the character that holds the item is flipped to when clearing the boss room. So if dead Lazarus has the item, tainted Lazarus will need to clear the boss room in order to get the additional item pedestal.
The Battery allows a second storage of Flip, allowing it to be used slightly more freely.
Book of the Dead: Bone orbitals and charmed Bonies spawned by Book of The Dead carry over through transforming and benefit either form of Tainted Lazarus.
Items like The Body, Cancer, and other drastic health increases allow Tainted Lazarus to tank large amounts of damage in longer boss fights if spread between forms.
Clicker: Becoming Tainted Lazarus by using Clicker can transform into him as a single character, without the Dead Tainted Lazarus pairing. This makes the run much easier, because it removes the complications the character usually has. He still carries Flip, but using it has no effect. (Note: Quitting the game and returning to the run will fix Flip)
Wisps created by Book of Virtues will remain after flipping between forms, and flipping to the form of Tainted Lazarus that is not holding the book will create a Flip wisp, even if Flip was not activated manually.
Large damage increases such as Cricket's Head or Synthoil are useful to get onto Tainted Lazarus's living form early to make up for his damage deficit.
Diplopia and Crooked Penny allow for potential duplications of strong items to get on both forms. Though keep in mind Devil Rooms or Angel Rooms would likely require the form holding them to be active upon entry. Locate the boss room, and if necessary, clear rooms until Flip is charged and you are playing as the form NOT holding Diplopia or Crooked Penny. Upon defeating the boss, Tainted Lazarus will Flip automatically, allowing the duplication of Devil or Angel Items, followed by flipping and taking the same item on the other form.
Esau Jr. is potentially Tainted Lazarus's best Item. This item acts differently for him and will take up both forms' Active Item slots, switching between the two of them on use. This effectively makes the item a one room charge Flip, allowing much more freedom with your playstyle, and if desired, focusing entirely on one form's build.
Having one form holdGuppy's Paw with a medium to large pool of heart containers can extend the potential time before needing to flip, extending your health bars even further.
Jumper Cables can allow for more use of one form's Active Item, or just more use of Flip.
Red Stew's damage bonus does not decrease when the form holding it is inactive.
Star of Bethlehem can be a good way to prevent one form from lagging behind the other power-wise.
Offensive Active Items that charge on a timer, especially Sharp Straw and Glass Cannon, can make up tremendously for one form that happens to be lagging behind. The Sharp Straw in particular, due to how it calculates damage, can make the weaker form much more offensively viable during end game boss fights.
In Greed Mode, if you take a hit with Crown of Light, flip, then flip back, the Crown of Light will be active again.
Old Capacitor is a great trinket to find. This will effectively turn Flip into a 12 charge item (due to only being active half of the time), however the increased spawning of Lil Batteries can allow more micromanagement to take place when a floor is finished, flipping between forms as needed.
Watch Battery is a good alternative, or even supplementary trinket to keep on the other form, as it increases chances of finding Lil Batteries as well.
Low charge Active Items with simple, effective uses, such as Shoop Da Whoop!, White Pony, or Spider Butt can be recharged within one large room, and can supplement the lack of power Tainted Lazarus is prone to.
High charge Active Items tend to be too high maintenance to get much use out of on Tainted Lazarus without an alternate plan to charge them.
While not exactly a bad choice, extra lives like Dead Cat or 1up! require a bit more thought to get use out of, due to only reviving the form holding it. If a 1up! is found, try to have the form with more heart containers pick it up to get the most use from it.
Any effect that only lasts for the current room, like Lusty Blood, loses almost all use while in Greed Mode, as Tainted Lazarus will switch forms per wave. In normal and hard mode, this is only true in situations such as Challenge Rooms or Boss Rush.
The achievement for unlocking Tainted Lazarus is called "The Enigma", a possible title for the character.
An "enigma" is commonly referred to as "a person or thing that is mysterious, puzzling, or difficult to understand.
Tainted Lazarus' wrists appear to be cut, suggesting self-harm or possibly even suicide.
This could be referencing the common strategy of intentionally dying as Lazarus to respawn as his stronger form, Lazarus Risen.
Tainted Lazarus's transition screen portrait.
Dead Tainted Lazarus's transition screen portrait.
When touching the white fire in Downpour/Dross II as either of Tainted Lazarus' forms, and then touching the white fire in the mirrored world as his other form, only the character used to pass through the mirror is able to return to their original form. The other character can freely pass to the mirrored world and back and remains as The Lost for the rest of the run.
Exiting the run during the second or third wave of a Challenge Room will forcibly flip Tainted Lazarus when the run is continued.
When one of the Tainted Lazarus' forms has Holy Mantle, after his other form flips into this one, Tainted Lazarus still won't gain the Holy Mantle effect until he leaves the room. Exiting to the main menu screen and then continuing the game will cause Holy Mantle to work correctly.
Each flip causes familiars to revert to how they would be when you first pick them up, meaning Lost Soul will be revived every time you flip back and that bums' payout will be reset.
If Tainted Lazarus uses Abyss and switches forms, the locusts generated disappear permanently.
If Tainted Lazarus uses Decap Attack and switches forms without recovering his head, upon switching back again that form's tears will appear from a point seperate from his body, until Decap Attack is used again.
When getting the Guppy transformation as either of Tainted Lazarus' forms, flies will not spawn but flight will still be granted to the Lazarus with the transformation.
When fighting Rotgut, whichever form was used for the first phase will temporarily become invisible after killing the third phase and Rotgut's death animation plays. This is purely visual.
If Tainted Lazarus switches form and then uses the Glowing Hour Glass while in the same room, the hourglass' charge will not be depleted. If Flip is fully charged, this can exploited for unlimited retries of a room.