Ebullism

Using forbidden torture magic, you boil a creatures blood.

Target creature you can see. Target creature initially takes 4d6 Pain damage, and must succeed a CON saving throw or fall prone immediately.

This spell can be maintained for 5 minutes, causing target creature to take the 4d6 damage at the beginning of each of your subsequent turns. A creature who makes it to the end of that 5 minutes dies with no chance of death saves.