Deadman's Wager

Binding Rite of the Godless Priest

You make a special pact, bet, or agreement that is infused with life-binding magic. When you cast the spell, you touch one willing creature, who must be within 5 feet of you throughout the casting. As part of casting this spell, both the target and you wager an equal number of hit dice, up to a maximum of 3. 

You set a condition for the spell to end, where one creature is the loser or the one who breaks the deal, and the other creature is the winner or the one who did not break the deal. The condition can be anything you choose, but it must be agreed upon by both the target and you, such as "whoever loses this race" or "I will never lie to you." Immediately after the condition is met, the spell ends and the loser expends the wagered hit dice. The loser takes necrotic damage which cannot be reduced equal to a roll of their wagered hit dice, and the winner gains temporary hit points equal to the damage taken. If the loser has not enough hit dice to expend, they instantly drop to 0 hit points. Both creatures know when the spell ends.

A dispel magic spell can end the spell early, targeting either the target or you.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, you can wager an additional 2 hit dice for each slot level above 1st. If you use a spell slot of 5th level or higher, the loser's hit point maximum is reduced equal to the necrotic damage taken. If you use a spell slot of 7th level or higher, the loser also gains 1 point of corruption. If you use a 9th level spell slot, the spell also summons an Avatar of Death in a space within 5 feet of the loser.