Black Elixir

Potion, legendary

This greasy black liquid acts as a poison when drunk or rubbed on flesh. Its unfortunate or foolish user takes 12d6 poison damage, or half damage with a successful DC 20 Constitution saving throw.

Its true power is revealed, however, when black elixir is smeared on a doorway or other portal; the magical oil acts as a forbiddance spell with a duration of one year. Unlike the spell, black elixir doesn't incorporate a password and only affects celestials and fiends. Typically, the creator of the elixir determines when the elixir is created whether the forbiddance effect deals radiant or necrotic damage.

Ingredients: the heart of a marut, the powdered horn of a ki-rin, and a vial of black mud taken from the bottom of a lake that at least one celestial or fiend has bathed in.