Cursed Swarm

Swarm. The Cursed swarm can occupy another creature's space and vice versa, and it can move through any opening large enough for a Tiny insect.

Soul decay.  If the Cursed swarm hasn't killed a creature, with an intelligence of 3 or higher, with its soul drain ability in a number of days equal to its intelligence, then its intelligence is reduced by 1 every day. The Cursed swarm dies if its intelligence is reduced to 0.

Bite. Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm's space. Hit: 9 (2d8) piercing damage and 9 (2d8) necrotic damage, or 4 (1d8) piercing damage and 4 (1d8) necrotic damage if the swarm has half of its hit points or fewer.

A creature reduced to 0 hit points by this attack is stable but poisoned for 1 hour, even after regaining hit points, and paralyzed while poisoned in this way.

Soul drain. The Cursed swarm targets one non-construct creature within its space. The target must succeed on a DC 15 Wisdom saving throw or its hit point maximum is reduced by 9 (2d8). The target has dissadvantage on this save if it is paralyzed. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.