Cephalid

Amphibious. The cephalid can breathe air and water.

Slippery. The cephalid has advantage on ability checks made to escape a grapple.

Semi-Amorphous. The cephalid can move through a space as narrow as 1ft wide by using half their movement.

Spellcasting. The cephalid is a 5th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 14, +6 to hit with spell attacks). The cephalid has the following Wizard spells prepared:
Cantrips (at will): Encode Thoughts, Mind Sliver, Minor Illusion, Lightning Lure

1st level (4 slots): Detect Magic, Disguise Self, Mage Armor, Thunderwave

2nd level (3 slots): Detect Thoughts, Levitate, Mirror Image

3rd level (2 slots): Counterspell, Dispel Magic

Multiattack. The cephalid makes one Tentacles attack and casts a cantrip.

Tentacles. Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 9 (2d6 + 2) bludgeoning damage.