There are supposedly over 100 special seeds in Rebirth. What is known about the seeds:
These special seeds will either add a filter to the game, change the graphics, or sound effects. These special seeds won't generate the same rooms every time like a normal seed would. It only applies the effect.
Any and all seeds disable achievements and unlockables, so do not use if you plan to make a serious run.
- In Afterbirth, any seeds which don't make the game easier or alter it drastically do NOT disable achievements. This means DARKNESS, FACEDOWN, MEDICINE, BRTLBONS, etcetera, all still leave achievements enabled, while PAC1F1SM, BASEMENT, LABYRNTH, and FREE2PAY all disable achievements.
Confirmed Special Seeds
PAC1F1SM - Enemies cannot be damaged, doors open by default, boss health slowly ticks down and you must survive until it reaches zero, grab the item, and get to the trapdoor. (Disables achievements in afterbirth)
BASEMENT - Infinite Basements (Disables achievements in afterbirth)
BRWNSNKE - Isaac infinitely leaves Poop behind him
FARTSNDS - Replaces Bomb, Tear, and Some other sound effects with farts!
B00BT00B - CRT television-like display
CHAMP1ON - More Champions spawn
DONTSTOP - You take damage if you stop moving (Will not disable achievements in afterbirth)
CHRSTMAS - Isaac wears a Santa hat.
H0H0H0H0 - see CHRSTMAS
FREE2PAY - You start with 69 coins, and everything costs money, be it pickups or items.
PAY2PLAY - Same as FREE2PAY
C0CKFGHT - All enemies are permanently Charmed.
BRTLBONS - You die in one hit.
FACEDOWN - Hides the Card faces from you.
MEDICINE - Pills remain unidentified after you use them. Hope you can remember them!
KEEPTRAK - No HUD. You can't see your keys, bombs, coins, health, pills/cards, trinkets, or the map. Good luck!
30MINITS - 30 minutes to complete the game. At 29:45 a timer starts ticking and after 30:00 Isaac is damaged once per second.
CUBEMEAT - all treasure room spawn cube of meat
LOST - Curse of the lost in all floors