Painless Death (5e Trap)
From D&D Wiki
This trap is located in a hemispherical room. This room has a large metal stick in the center. The stick has a magical field around it, which is tripped when X% of the party enters it, as chosen by the DM, and light emerges from the stick which deals 1 fire damage per turn on a failed DC 20 Dexterity save, if this happens for 3 turns, it burns a small, unnoticeable, magic symbol into the person. This part of the trap can be disabled in 6 turns of DC 10 Arcana checks. If a character has the magical symbol inscribed on them they take 1d4 force damage per turn, but are unconscious of the fact. A successful DC 15 Arcana check can notice this. A DC 10 Medicine check can remove the rune, but only once it is noticed. If the rune reduces them to zero, the character dies immediately.