Cross Necklace

The Cross Necklace is an accessory that doubles the duration of the brief invincibility after taking damage from 2/3 of a second to 1 1/3 seconds. It has no effect in PvP. It is dropped by mimics. It is useful in fights where the player gets hit in rapid succession, such as against The Destroyer as well as Skeletron Prime.

Trivia
This may be a reference to the first Castlevania, having nearly the same image and making you invincible for a short duration.

(Except the Cross Pendant cleared the screen of enemies, rather than increasing your mercy invulnerability)