Ouroboros Rapier

Weapon (rapier), very rare (requires attunement)

You gain a +1 bonus to attack and damage rolls you make with this magic weapon. Whenever you roll an 8 on the damage die for an attack with this weapon, you can immediately roll another 1d8 and add it to the total. In addition, when you land a critical hit with the rapier against a creature, it must make a DC 14 Constitution saving throw. On a failure, the creature is poisoned for 1 minute and takes 3d6 poison damage. On a success, the creature is not poisoned, and the poison damage is halved.

The relentless strikes. The cold, sharp piercing. The venom. If this is what infinity is, I don't like it.