The Ninth World (Homebrew), D20 Modern Arcana
Swarm 2.0. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny monstrosity. The swarm can’t regain hit points or gain temporary hit points. A swarm takes half damage from attacks that use an attack roll and double damage from effects that require it to make a saving roll. Creatures that are not swarms are impaired while they remain in the swarm’s space or within 5 feet of it and have disadvantage on attack rolls and Constitution saving throws to maintain concentration. The area of the swarm is considered difficult terrain.
Rip and Tear. The swarm automatically does 14 (4d6) piercing damage to any creature sharing its space or adjacent to it. This damage can only be healed naturally and not through magic. target must also succeed on a DC 12 Constitution saving throw or be poisoned. At the end of each of the poisoned target’s turns, the target takes 3 (1d6) poison damage. Whenever the poisoned target takes fire damage, the target can repeat the saving throw, ending the effect on itself on a success. If the poisoned target ends its turn with 0 hit points, it dies and a fully formed Grendalspawn Hunter emerges.
Grendalspawn lair