Army of Filth

This spell calls forth a deluge of verminous, biting creatures, usually of the most vile variety (leprous rats, large ugly spiders, centipedes, and the like); the swarm fills a 20-foot radius circle on the ground at a point you can see in range.

A creature that starts its turn in the area of the swarm takes 1d4 points of piercing damage as the creatures swarm over their body; for every foot a creature moves through the swarm, they take this damage as well.

When a creature ends its turn after having taken damage from this spell, it must succeed on a Constitution saving throw, or be afflicted with a terrible illness. The creature repeats the saving throw at the end of each of its turns if it fails the first saving throw. For each failed saving throw, including the first one, the creature suffers increasingly terrible effects; on the first failure, they are poisoned; a second failure causes them to be blinded; finally, a third failure the creature's movement speed it halved as it is wracked with horrible, clenching pains.

The illness can be ended early by an effect that removes diseases, when you lose concentration on the spell, or if the creature succeeds on as many saving throws as it has failed. For example, a creature at the third tier of effects would have to succeed three times to rid itself of the illness.