Momentary Peril

This curse is obtained by interacting with an orb found in the first room a floor. The curse will set you to 1 HP and shows a number at the bottom. This number indicates the number of rooms that must be entered to remove the curse. Each room entered will lower the number by one and after the number goes to zero the curse will be removed yielding a free random relic.

This curses is unique in the sense that it is the only curse with a "Hex border", i.e. the curse cannot be removed by normal means such as
 * at Altars

The 1 HP does not remove armor points or effects like nullstone, similar to the Hex so it doesn't necessarily require the player to not get hit at all.

The conditions for the orb to appear are not currently known, but it seems to be guaranteed to appear in a normal Undermine run and potentially only after you have obtained a certain power level or number of relics/blessings.