Virulent Bloat

You target one creature within your sight that you have damaged with a slashing or piercing weapon in the last hour and force them to make a Constitution saving throw. On a failure, parts of their body swell up with large pustules of foul-smelling liquid and take 1d8 necrotic damage. 

Afflicted targets have their movement speed halved and gain disadvantage on Strength and Dexterity checks. They take 1d4 necrotic damage whenever they attack. If the target hits 0 hit points while afflicted, the pustules explode and deal 3d6 poison damage to all creatures within a 10 foot radius. If the pustules are unexploded after 1 minute or if the caster loses concentration, the affliction begins to subside and the creature returns to normal.

When cast at 4th level, any creatures damaged by the erupting pustules must make a 13 DC Constitution saving throw. Upon failure, they become afflicted with the same bloated condition unless the original caster stops concentrating.