Well Spirit

Water Form. The well spirit can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Freeze. If the well spirit takes cold damage, it partially freezes; its speed is reduced by 20 feet until the end of its next turn.

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

Spider Climb. The well spirit can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Runic Vulnerability. The well spirit is vulnerable to weapons and ammunition carved with a particular rune of protection. Additionally, it cannot cross a line of stones engraved with this symbol, as if a Wall of Force were in place.

Multiattack. The well spirit makes two slam attacks. 

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

Whelm (Recharge 4–6). Each creature in the well spirit's space must make a DC 15 Strength saving throw. On a failure, a target takes 13 (2d8 + 4) bludgeoning damage. If it is Large or smaller, it is also grappled (escape DC 14). Until this grapple ends, the target is restrained and unable to breathe unless it can breathe water. If the saving throw is successful, the target is pushed out of the elemental's space.

The well spirit can grapple one Large creature or up to two Medium or smaller creatures at one time. At the start of each of the elemental's turns, each target grappled by it takes 13 (2d8 + 4) bludgeoning damage. A creature within 5 feet of the Well Spirit can pull a creature or object out of it by taking an action to make a DC 14 Strength check and succeeding.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the Well Spirit, incapacitated, or restrained. Hit: 10 (2d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The well spirit regains hit points equal to the damage dealt.

Fluidic retreat. The Well Spirit disengages from combat automatically if it makes no attacks on its turn and flees from combat.

Slurp. The Well Spirit can drink the blood from a recently killed creature to recover 3d6 hit points. 

Liquid dodge. Once per turn the well spirit can add +5 to its AC against a melee attack by becoming a shapeless fluid.