Armor Class 16 (natural armor)
Hit Points 112 (15d8 + 45)
Speed 30 ft.
Damage Resistances [object Object]
Senses darkvision 60 ft., passive Perception 10
Languages Common
Challenge 9
Psychic Leech. At the start of each of the Lonely's turns, each creature within 5 feet of it must succeed on a DC 15 Wisdom saving throw or take 10 (3d6) psychic damage.
Thrives on Company. The Lonely has advantage on attack rolls while it is within 30 feet of at least two other creatures. It otherwise has disadvantage on attack rolls.
Multiattack. The Lonely makes one harpoon arm attack and uses Sorrowful Embrace.
Harpoon Arm. Melee Weapon Attack: +7 to hit, reach 60 ft., one target. Hit: 21 (4d8 + 3) piercing damage, and the target is grappled (escape DC 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. Each creature grappled by the Lonely must make a DC 15 Wisdom 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 30 feet straight toward it.
Mordenkainen's Tome of Foes, p. 232