Gives you a hard timer of 30 minutes to beat the game. Isaac is completely invisible, however you can still see the light effect surrounding him. Note: Seeds are all 8 characters long and all alphanumeric characters are allowed except the letters I, O, U and V, because they look too much like other characters.
This page highlights all the secret seed codes which the developers have added to the game and what each seed does. Rebirth has over 4 BILLION possible seed combinations, however there are 50+ pre-assigned seeds which have unique effects on the game.
You won't be unable to unlock secrets while playing seeded runs, however they allow you to share the seed with your friend and play the same run together. In The Binding of Isaac: Rebirth, the seed determines every item, enemy, floor layout and room in the run. Isaac Cheat Sheet Secret Seed Codes (Rebirth)
