Phosgene Flail

Rod, very rare (requires attunement)

This heavy baton is perforated with dozens of tiny holes at one end, from which leak tendrils of toxic mist. As an action, you can command these noxious vapors to coalesce into a semi-solid ball of gas that can be wielded as a +1 flail that deals an additional 1d4 acid damage and 1d4 poison damage on each hit. If you strike a creature twice in the same round with the phosgene flail or score a critical hit with it, the target must make a DC 14 Constitution saving throw or become poisoned. The target can make a new saving throw each round at the end of its turn to end the poisoned effect. Creatures that do not breathe cannot be poisoned by the phosgene flail.

When you cast a spell that deals acid or poison damage with the phosgene flail in hand, you gain a +1 bonus on spell attack rolls and the save DC is increased by 1. Once per day you can use the rod to cast either cloudkill or stinking cloud, with a saving throw DC of 17 for either.