Curse of Yig

7th-level transmutation

Casting Time: 1 action

Range: Special

Components: V, S, M (a snake tail and a humanoid hand)

Duration: Instantaneous

You call down Yig’s displeasure upon a humanoid who has betrayed a secret that you directly shared with it. You can cast the spell on any target creature on the same plane of existence as long as you have reason to believe the target betrayed your secret. (If you are wrong in your belief, the spell is wasted to no effect.) The target must make a Constitution saving throw. On a successful save, the target forever after automatically succeeds on the saving throw against this spell when you cast it. On a failed save, one of the target’s limbs (usually an arm) transforms into a viper and bites the target. The viper is a poisonous snake with hit points equal to half the target’s maximum.

The first time the viper bites the target who has betrayed your secret, it automatically hits, dealing 1 piercing damage and 7d10 poison damage. The target automatically becomes poisoned and its flesh bloats purple around the wound, hemorrhaging throughout its body (but leaving the snake unharmed). Each time the target begins its turn with this poisoned condition, the target must succeed on a Constitution saving throw. If the saving throw fails, the target takes another 3d10 poison damage. On a successful save, the target takes no damage and the poisoned condition ends.

Roll initiative for the snake, which has its own turns. As long as it is attached to the target of the spell, the snake has advantage on attack rolls to hit the target, while the target has disadvantage on attack rolls to hit the snake.

The snake knows everything the target knows, but its ability to use that information is limited by its Intelligence.

Any damage, healing, or spell targeting the snake or victim while they are attached applies to the other as well unless the other is immune. The viper is immune to poison damage and to the poisoned condition. If the target of the spell is reduced to 0 hit points, the snake detaches and moves to attack the nearest creature within 10 miles that has learned the secret, continuing on a rampage of murder until the snake is killed or no targets are left within range.

The snake then becomes a normal poisonous snake.

When the viper’s hit points are reduced to 0, it dies and detaches from the target. A regenerate spell or similar ability can restore the original limb after the viper has detached or been cut off, but only if the caster includes as an additional material component: an offering to Yig worth at least 1,000 gp, which the spell consumes. Without this offering, a regenerate spell causes the limb to grow back in the form of a new attached viper, which attacks the target again. If the target dies from the snake’s attack or by any other cause while missing the limb, the target cannot be raised from the dead by the raise dead spell. Only a more powerful spell such as resurrection or wish can bring the target back to life.