Nidoplate (5e Equipment)
From D&D Wiki
Armor (plate), very rare (requires attunement)
If you are hit by a melee attack while wearing this armor, you can use your reaction to cast lingering poison (DC 15) on the attacking creature. When cast in this way, the spell does not require your concentration. Once a creature makes the saving throw for this spell, regardless of success or failure, it automatically succeeds on all further saving throws against this effect.