Kelpie

This malevolent fey feeds on the flesh of humanoids.  The kelpie appears as a black horse with black eyes. It can also assume the form of a humanoid, often appearing as a disheveled man in need of assistance or a woman who bids its target to come close. Once its target is within range, the kelpie will drop its act and savagely attack.

    Water Lurker. The kelpie prefers to live near a body of water, particularly a lake or pond. As it can breathe both air and water, the kelpie will spend its resting time underwater and out of sight. It rises when it senses prey or to hunt.

    Malevolent Trickster. The kelpie targets lone travelers or unsuspecting victims such as children. It can appear as a horse or as a human, using its various abilities to deceive and lure prey as close to the water as possible.

    If a creature enters the water, the kelpie will attempt to drown the creature. When possible, it will tempt a victim to try to ride it, plunging itself and the rider deep into the water when it complies.

Kelpie

Large fey (shapechanger), neutral evil

Armor Class 15
Hit Points 114 (12d10 + 48)
Speed 60 ft., swim 60 ft.

STRDEXCONINTWISCHA
22 (+6)14 (+2)18 (+4)13 (+1)13 (+1)14 (+2)

Saving Throws Str +10, Con +8, Cha +6
Skills Deception +6, Insight +5, Perception +5
Damage Resistances cold, fire; bludgeoning, piercing, and slashing from nonmagical weapons
Condition Immunities poisoned
Senses blindsight 60 ft., passive Perception 16
Languages Common, Sylvan
Challenge 8 (3,900)

Amphibious. The kelpie can breathe air and water.

Shapechanger. The kelpie can us its action to polymorph into a humanoid. Aside from size and attacks, it retains its own statistics. It can assume its true form by taking a bonus action.

Innate Spellcasting. The kelpie’s spellcasting ability is Charisma (spell save DC 14). The kelpie can innately cast the following spells, requiring no material components:

At will: dancing lights, silent image

3/day:  control water, darkness

Innate Manifestation. The kelpie’s manifesting ability is Charisma (power save DC 14). The kelpie can innately manifest the following powers:

At will: body control (up to 6 power points), chameleon (up to 2 power points; in water only), mind probe (up to 3 power points), telempathic projection (up to 4 power points)

3/day: accelerated healing (up to 7 power points), decay (up to 4 power points), psychic domination (up to 8 power points)

Trapped Steed. A living creature that mounts a Kelpie must make a DC 16 Charisma saving throw to dismount it.

Mimicry. The kelpie can mimic the voice of a humanoid. If it has heard a specific voice, it can attempt to mimic it by succeeding a Charisma (Deception) check against the Wisdom (Insight) of a creature which can hear it and recognize the voice.

Actions

Multiattack. The kelpie makes two hoof and one bite attack.

Hoof. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 10 (1d8 + 6) piercing damage.