Crawling Blood Swarm

Blood Frenzy. The swarm has advantage on attack rolls against any creature that doesn't have all its hit points.

Magic Resistance. The swarm has advantage on saving throws against spells and other magical effects.

Spider Climb. The swarm can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny insect. The swarm can't regain hit points or gain temporary hit points.

Multiattack. The crawling blood swarm makes two Torturous Mandibles attacks.

Torturous Mandibles. Melee Weapon Attack: +9 to hit, reach 0 ft., one target in the swarm's space. Hit: 15 (4d4 + 5) piercing damage plus 7 (2d6) necrotic damage, or 12 (3d4+5) piecing damage plus 3 (1d6) necrotic damage if the swarm has half of its hit points or fewer.

Blood Pollution (Recharge 5-6). One non-construct non-undead creature within 5 feet of the crawling blood swarm that has half its hit point maximum or less remaining must succeed on a DC 17 Consitution saving throw or another crawling blood swarm appears in an unoccupied space within 5 feet of that creature.