Well-Beast

Player-Scaling. Add +1 to all dice rolled for damage and HP equal to the highest level among all player characters that are in combat against the Well-Beast. Also, increase the Well-Beast's CR by 1 equal to the highest level among all player characters in combat against the Well-Beast. After the Well-Beast has been defeated, each player in combat with the Well-Beast gets 3 gp for each level their character has.

Spellcasting. The Well-Beast is a 5th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 10, +0 to hit with spell attacks). The Well-Beast has the following Warlock spells prepared when its CR is 5 or above:
Cantrips (at will): Infestation, Minor Illusion, Poison Spray 
1st level (4 slots): Cause fear, Hex, Witch bolt

Claws. Melee Weapon Attack: +1 to hit, reach 5 ft., 1 target. Hit:(0d6) Slashing damage. 

Well-Beast Bile. RangedWeapon Attack: +0 to hit, range 15/25 ft., 1 target. Hit:(0d8) Acid damage.

Claw Bile. The Well-Beast vomits onto its claws, making its next melee attack on that turn deal an extra 0d4 acid damage (Increase the damage dice equal to its CR).