Darkvision. Allows creature to see in the dark. Dim light is perceived as Bright light and No light is perceived as dim light. This ability ranges to 60ft.

Survivor. Aboreth regains 10 hit points at the start of its turn if it has atleast 1 hit point but fewer hit points than half its hit point maximum.

Spellcasting. 

Cantrips (at will): [Venomous Spines]

2nd level (1 slots): [Choking Smog]

3rd level (2 slots):  [Spore Burst], [Major Image], [Fly]

6th level (2 slots): [Fume Form], [Fire-Ball]

Mille-pede stragulation. Melee Weapon Attack: +10 to hit, reach 30 ft., 2 target max. Hit: 10 (1d20 + 5) [Force+Poisoning] damage.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit 11 (1d6 + 4) [Piercing] damage.

Aboreth the Devourer can take 3 legendary actions, choosing from the actions below. Only one legendary action can be used at a time and only at the end of another creature's turn. Aboreth regains spent legendary actions at the end of each turn.

Command Ally. Aboreth Commands 1 ally it can see within 30 ft of it. If the target can see and/or hear Aboreth, the target can make 1 weapon attack move at Aboreth’s command and gains advantage on the attack roll.

Frightening Gaze (Costs 2 Actions). Aboreth fixes its gaze on one creature it can see within 10 feet of it. The target must succeed on a DC 18 Wisdom saving throw against this magic or become frightened for 1 minute. The frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to Aboreth’s gaze for the next 24 hours.

Disrupt Life (Costs 3 Actions). Each living creature with 20 ft of Aboreth must make a DC 18 CON saving throw against his magic, taking 21 (6d6) necroitic damage on a failed save or half as much on a successful one.