Civil. The Gatekeeper will often attempt to ward away intruders before attacking unless provoked.

Ethereal Sight. The Gatekeeper can see 20 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.

Detect Life. The Gatekeeper can magically sense the presence of creatures up to 1 mile away that aren’t undead or constructs. It knows the general direction they’re in but not their exact locations.

Sunlight Sensitivity. While in sunlight, the Gatekeeper has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Life Drain. (2/day) Melee Weapon Attack: +6 to hit, reach 10 ft., one creature. Hit: 11 (2d8 + 3) necrotic damage. The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Tentacles. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage. If the target is a creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the Gatekeeper can't use it's tentacle on another target. It can have up to 4 creatures grappled at a time, but can no longer move or deal damage once all of it's tentacles are in use.

Acid Spray (Recharge 6). The Gatekeeper spits black sludge in a line that is 30 feet long and 5 feet wide, provided that it has no creature grappled. Each creature in that line must make a DC 13 Dexterity saving throw, taking 10 (3d6) acid damage on a failed save, or half as much damage on a successful one.