Ankh

Ankh is a passive item.

Effect

Resurrection items activate in a set order. Ankh is 6th / 8th in this order after Soul of Lazarus, 1up!, Dead Cat, Lazarus' inherent extra life ( swap the order of the previous two), Inner Child, Guppy's Collar, and Lazarus' Rags.
Like other items that grant an extra life, but respawn Isaac as a different character, all completion marks earned from the moment of death onward will count towards the new character, not the old.
During The Beast fight, Isaac respawns in the place he died.

Tainted ???: Revives as himself, rather than normal ???. Effectively acts as a 1up!.

The Ankh is an Egyptian hieroglyph that means "life".
It's possible that the resurrection as ??? is a reference to mummies, often depicted as undead in pop culture.
In the original The Binding of Isaac, the Ankh gave Isaac eyeshadow, like the one Eve wears.
This item shares a pickup quote with Guppy's Collar.

Bug!	PS4 When downloading a synced file where player 2 was revived as ??? before the character is unlocked, Player 2 will become Player 1 and be loaded as Isaac.