Boss Rush

The Boss Rush is an optional room that becomes available by beating Mom in The Depths, Necropolis or Dank Depths within 20 minutes, or beating her in  Mausoleum or  Gehenna within 25 minutes.
 * It can also be accessed by having the equipped or with  active.

In the center of a large room, there will be 4 random items to choose from. Picking up an item will cause the other three to disappear and start the Boss Rush. Completing the Boss Rush will also reward 1 more item, separate from the 4 seen when first entering the room.

There is a chance that there will only be 2 items to choose from. Additionally, items may be trapped behind rocks or spikes. Items may appear in the corners of the room instead of the center.

There is a small chance that there will be 8 items to choose from.

Waves
The Boss Rush consists of 15 waves with 2 bosses per wave. The next wave does not spawn until both bosses from the previous wave have been killed, and any lingering monsters from previous waves will not prevent the next wave from spawning.

Occasionally, all the bosses fought in the Boss Rush will be their Champion variants, if they have one.

Rebirth
All 15 waves are predetermined.
 * Wave 1: Blighted Ovum and Gurdy Jr.
 * Wave 2: Carrion Queen and The Wretched
 * Wave 3: Peep and The Duke of Flies
 * Wave 4: Blastocyst and Loki
 * Wave 5: Steven and Gemini
 * Wave 6: 3x Larry Jr. and Fistula
 * Wave 7: C.H.A.D. and Monstro
 * Wave 8: Gurdy and Chub
 * Wave 9: Monstro II and Gish
 * Wave 10: Widow and Pin
 * Wave 11: 3x The Hollow and The Husk
 * Wave 12: Mask of Infamy and The Bloat
 * Wave 13: Pestilence and Famine
 * Wave 14: Death and War
 * Wave 15: The Fallen and The Headless Horseman

Afterbirth
All 15 waves are randomized, with each wave spawning two random bosses from the list. Each boss can only appear once per Boss Rush, meaning that out of all the 50 bosses Isaac will face 30 of them.

Note that all of the chapter 4 bosses (except Blastocyst and Death) are not present in the Boss Rush:
 * Scolex (62.1)
 * Conquest (65.1)
 * Lokii (69.1)
 * Teratoma (71.1)
 * Daddy Long Legs (101.0)
 * Triachnid (101.1)
 * Mama Gurdy (266.0)
 * Mr. Fred (270.0)

Furthermore, some other bosses are not present in the Boss Rush:
 * All Final Bosses
 * All Sins and Super Sins
 * Krampus (81.1)
 * Fallen Uriel (271.1)
 * Fallen Gabriel (272.2)
 * All Afterbirth † Bosses
 * All Repentance Bosses

Tips

 * The player's playtime can be checked at any moment by pressing TAB (or long-pressing SELECT on the controller), enabling Extra HUD in the options menu, or by simply looking down to the bottom screen on the 3DS version, where it is shown on the map.
 * Each boss will drop Red Hearts on normal difficulty. can turn these into Soul Hearts, providing loads of extra health.
 * Take out Carrion Queen quickly, as its red poops will remain for the rest of the boss rush. They can be advantageous though, as other bosses can also take damage from these poops. In particular, bouncing bosses like The Hollow and The Husk can quickly kill themselves on contact.
 * All bosses with laser attack could severely hurt or even one-shot-kill each other, especially  The Gate. It may be beneficial to focus on the other boss before killing them.
 * Wave 9 can be quite difficult as Monstro II (he always spawns on the left side) can catch the player off guard with an offscreen laser. The black creep left by Gish will remain until he is killed.
 * The Bloat + Mask of Infamy is one of the hardest waves, but The Bloat can be baited into destroying both the heart and the mask itself of the Mask of Infamy with its lasers.
 * Any Teleport effect (such as cards, activated items, Telepills, etc) can be used to leave the Boss Rush. This can be used to gain one item without having to fight any bosses.
 * After unlocking, Marked Skulls will always appear on Depths II and will always drop , providing a guaranteed way to escape Boss Rush.

Trivia

 * Despite battling bosses from the floors before, a few bosses from will spawn such as Blastocyst and Death.
 * Despite the Timer seen between level transitions appearing, Boss Rush cannot be done the first time the player defeats Mom. Instead, the game will cut directly to the Epilogue.
 * Armageddon is the theme for the Boss Rush added in The Binding of Isaac: Repentance.