Mystery Egg

Mystery Egg is an unlockable passive item added in The Binding of Isaac: Afterbirth † (Booster Pack #5).

Effect

Spawns an egg familiar, which cracks and turns into a random friendly monster when Isaac is hurt.
- The egg regenerates upon entering a different room.
- The usefulness of the enemy depends on how many consecutive rooms have been cleared without getting hurt.

Familiars that follow behind Isaac have a certain priority in how close their position is to him over other familiars, with Mystery Egg having the lowest priority at 6th and thus being farther from Isaac than familiars of different priorities.
Mystery Egg will activate when taking non-hostile damage (such as from a Blood Donation Machine or Curse Room door).
For Greed Mode and Greedier Mode, the usefulness of the enemy is based on waves cleared rather than rooms.
The list of available monsters is as follows:


Number of rooms / waves cleared	Monsters
0 to 1 rooms cleared without taking damage	Fly, Attack Fly, Maggot, Spider, Dip, Embryo, Nerve Ending
2 to 4 rooms cleared without taking damage	Moter, Pooter, Swarm, Level 2 Fly, Full Fly, Horf, Sucker, Dart Fly, Big Spider, Guts, Hopper, Gusher, Round Worm, Mask + Heart, Gush
5 to 7 rooms cleared without taking damage	Squirt, Ragling, Homunculus, Level 2 Spider, Small Baby Long Legs, Wall Creep, Night Crawler, Ulcer, Ministro, Mulligan, Hive, Nest, Flaming Hopper, Gaper, Cyclopia, Skinny, Dople, Globin, Maw, Psychic Horf, Tumor, Fatty, Half Sack, Clotty, Charger, Brain, Conjoined Spitty, Buttlicker, Host, One Tooth, Baby, Dukie, Black Globin's Body, Black Globin's Head, Para-Bite
8 to 10 rooms cleared without taking damage	Splasher, Boom Fly, Keeper, Fat Bat, Psy Tumor, Imp, Fred, Cod Worm, Lump, Swinger, Bony, Black Globin's Body, Nulls, Begotten, Knight, Bone Knight, Blubber, Conjoined Fatty, Oob, Leech, MemBrain, Wizoob, Mr. Maw, Flesh Death's Head, Mushroom, Blind Creep, The Thing, Blister, Leaper, Hanger, Poison Mind, Fistuloid, Tar Boy
11 to 13 rooms cleared without taking damage	Gurgle, Walking Gut, Black Bony, Black Maw, Camillo Jr., Eye, Roundy, Mom's Hand, Mega Clotty, Swarmer, Fat Sack, Mobile Host
More than 13 rooms cleared without taking damage	Red Ghost, Rage Creep, Vis, Gurglings, Dinga, Portal

BFFS!: The egg grows in size. Enemies spawned will be a champion variant.
- Even if the enemy spawned cannot be a champion under normal circumstances, they will become a champion.

Ace Cards: Friendly monsters are still affected by Ace Cards.
Blood Puppy: Either angry version of Blood Puppy will target friendly enemies as well and can drag them along with it when it charges them. Blood Puppy cannot damage friendly enemies however friendly enemies can damage Blood Puppy despite not targeting it outright.
Broken Modem: Broken Modem also affects friendly enemies and projectiles created by them.
Eraser: Enemies erased by Eraser will never show up again, hindering Mystery Egg should it spawn said removed enemy.
Ghost Bombs / Hungry Soul / Purgatory / Vade Retro: Explosions from ghosts can kill friendly enemies.
Glowing Hourglass: Each use of Glowing Hourglass that does not move Isaac to a different room than the one he is currently in (such as the starting room of a new floor) will duplicate friendly enemies. (This is possibly a bug)
Holy Mantle: Losing the Mantle will not cause a friendly enemy to spawn.
Tick: Friendly enemies start with less health if they have 60HP or higher.

Bug!	When using Extension Cord with Mystery Egg, after the egg turns into a friendly enemy, the electricity shoots out into a random corner of the room.

Bug!	When the egg turns into a friendly Heart ( Mask + Heart) will sometimes act as the heart companion for a mask already in the room resulting in an unbeatable room.