The LonelyMedium monstrosity
Psychic Leech. At the start of each of the Lonely’s turns, each creature within 1 meter of it must succeed on a Difficulty 15 Will saving throw or take 10 (3d6) psychic damage.
Thrives on Company. The Lonely has advantage on attack rolls while it is within 5 meters of at least two other creatures. It otherwise has disadvantage on attack rolls.
Harpoon Arm. Melee Weapon Attack: +7 to hit, reach 12 m. Hit: 21 (4d8 + 3) piercing damage, and the target is grappled (escape Difficulty 15) if it is a Large or smaller creature.
The Lonely has two harpoon arms and can grapple up to two creatures at once.
Sorrowful Embrace (1/turn). Each creature grappled by the Lonely must make a Difficulty 15 Will saving throw. A creature takes 18 (4d8) psychic damage on a failed save, or half as much damage on a successful one. In either case, the Lonely pulls each creature grappled by it up to 5 meters straight toward it.
No concoctions, maneuvers, or spells
All creatures in this system should have maneuvers or spells. You should add some maneuvers or spells to this creature. If you do so, please make a suggestion on github so I can finish adding maneuvers and spells to all creatures via https://github.com/mlenser/kryx-rpg-issues/issues/9.