Flush! - When a room is cleared with Flush! a bonus will be awarded.

Door Stop - If the player has failed Death's List but hasn't cleared the room, leaving and re-entering will reset the list.

Upon entering a room, a skull appears over an enemy's head.

Killing the marked enemy will cause the skull to move to a different enemy.

If all enemies in the room are cleared in the order they are marked, Isaac gains one of the following rewards at random:

A Soul Heart, a pill, a Key, a Bomb or a Nickel.

+0.2 speed, +0.5 range, +0.5 tears, +0.2 shot speed, +1 flat damage or +1 luck.

If a non-marked enemy is killed, the skull will disappear and no bonuses will be granted for that room.

Grants a pickup or stat upgrade if all enemies in a room are killed in the order they are marked.

The Binding of Isaac Wiki

Death's List