In the Prisoners' Quarters, there is a chance for a lore room to spawn with a Prie Dieu. Examining this altar will drop this item. Picking the skill up will instantly unlock it.
The damage from this skill does not trigger death by curse or reset flawless killstreak, but can still kill the user through damage alone.
However, if the Custom Mode Modifier "Poison Wounds" is active, activating Face Flask will Poison you, causing you to die from curses or lose your killstreak.
This item is a reference to the game Blasphemous, and that game's protagonist's unusual way of applying healing flasks.
The flavor text references the ending of Diablo, where the protagonist also inserts a red object into their forehead, similarly causing a lot of problems for everyone.