On use, the character, including stats and initial items, is reset and teleported to a unique Bedroom that acts like an I AM ERROR room. It has a Bed, two random Chests, two Sacks, and a random Trinket. The only way out is through the beam of light.
This includes resetting the character to the initial one even if they had transformed due to items such as Ankh.
For every item removed using Genesis, there will be a set of three items from the removed item's pool to choose from, each set appearing one after the other.
If used on one of the final floors of the game, such as Chest, Dark Room, or The Void, the beam of light will be replaced with a Crawl Space that leads Isaac back to the starting room of the floor he used it on. In addition, the aforementioned chests will include items. These items will not disappear along the "choice" pedestals. The Mega Satan Door will remain open if it was opened earlier.
If used during The Beast's fight, it will merely teleport Isaac to the middle of the room, removing no items from him.
Genesis will remove all currently active curses, meaning Curse of the Blind will not affect the item pedestals present inside the room.
If used as Jacob and Esau or Tainted Lazarus, or in Co-op, both of the characters will get all their items removed, but they'll only be presented with replacement pedestals for items removed from whomever used Genesis, likely resulting in a net item loss.
Jacob and Esau cannot take two items at once from the choices offered by Genesis.
Tainted Bethany will lose all of her Lemegeton wisps upon use, but not her red heart charges. This allows her to quickly gather power again after the reset.
If used as Eden, a new set of items and stats will be generated. The first random passive item Eden starts with is rerolled immediately with no choice of three.
If Isaac uses Genesis while in Downpour / Mines / Mausoleum or their variants, the trapdoor can send him back to the regular floors unintentionally.
Using Genesis after defeating Mom's Heart on Mausoleum II will teleport Isaac to a room that is empty besides a trapdoor leading to Corpse I and the three item selections.
Since Genesis resets Isaac's health, it is one way to remove Broken Hearts.
Crawl Spaces can be generated and entered inside the Genesis room, but upon leaving them, Isaac will be returned to the floor Genesis was used on instead of returning to the room.
As Genesis cannot remove Damocles, it will work in the Genesis room as well, meaning that each set of choice pedestals will spawn an extra set.
Echo Chamber: The consumable pool is reset when Genesis is used, so if it's re-obtained after using Genesis, the previously stored consumable types won't be activated.
Eden's Blessing: Using Genesis after collecting the blessing during the run will spawn Isaac in the Genesis room already holding a random item. This consumes Eden's Blessing, and the next run Isaac starts will not begin with a random item.
Esau Jr.: Using Genesis as Esau Jr. will revert back to the original character.
Glowing Hour Glass: Using it works as intended and will give back all previous stats, items, pickups, and health.
Magic Skin: Genesis does not remove the downside of Magic Skin; any of the choice pedestals can be replaced by Magic Skin.
Pandora's Box: Using it inside the Genesis room will spawn The Bible. Picking it up does not affect the choice pedestals.
Genesis refers to the first book in the Bible, and is an Ancient Greek term for "beginning". The item description refers to the very first words in the Book of Genesis.
In Challenges, using this item will restore Isaac to the original set of items appropriate for the challenge, but with duplicates of the Passive items. This is specifically because Isaac's starting items in Challenges are prevented from being removed or changed.
If Isaac had multiple copies of any given item before using Genesis, the number of items presented will be equal to the number of copies of the item squared. Example: if 4 copies of The Sad Onion are removed, 16 sets of items will be presented in return.
Using Genesis as Jacob and Esau causes the link between characters to break: Esau can die without killing Jacob, and speed does not get synced between them. Additionally, if the run is exited and resumed, another controller is necessary to continue play.
Using Genesis as Tainted Forgotten causes the link between characters to break: Tainted Soul will now die in a single hit regardless of health remaining, as without that special connection, it functions identically to The Lost. Quitting and continuing will repair the link and fix the health issue, but objects taken before will be stuck on Tainted Soul. Stat changing items stuck on Tainted Soul are unusable and HP changes have no effect, but familiars, on-hit items, and other non-stat changing items will work, but be connected to Tainted Soul rather than Tainted Forgotten.
Using Genesis as Dead Tainted Lazarus will sever the link between him and Tainted Lazarus: the latter will be removed forever, removing the downside of playing Dead Tainted Lazarus at the expense of half of the items.
After using this item in the run, the trapdoor on Womb II will lead to the Cathedral instead of Sheol.
Using Genesis immediately after taking Dad's Note will teleport Isaac to the Genesis room, then to the start of the ascent with no items except those that cannot be removed (Dad's Note, Key Pieces, etc.).
If the run is exited and resumed while inside the Genesis Room, all three items in the current choice will become safe to pick and the next choice of three items will stack over them. The effect is stackable for every item Isaac had.
When Tainted ??? uses Genesis, he will appear in the Genesis room with a second Hold in his active item slot, in addition to the 'fixed' Hold in the card/pill/rune slot.
Using Genesis in the Mirrored World in Downpour / Dross II will result in a softlock, as the ray of light to get out of the room will not appear.