"Nuke Fish"

A herring with a green glow and the ability to explode with immense power, forcing everything within 1 mile to make a DC 16 Constitution saving throw. On a failed save, the creature takes 4d6 Necrotic damage + 3d10 Poison damage and is poisoned for 1 year. On a successful save, the creature is poisoned for 10 days. On a failed or successful save, the creature takes 30d10 force damage - 1d10 per 50 feet distance from the Nuke Fish. This damage treats both resistance and immunity as resistance. The Nuke Fish is then destroyed.

(lol just made this for fun)