Cosmic Eel

Multiattack. The eel can use its Frightful Presence. It then makes two attacks: one with its bite and one with its tail.

Wormhole. The eel teleports to anywhere on the same plane as it is on. 

Bite. Melee Weapon Attack: +12 to hit, reach 20 ft., 1 target. Hit: 19 (2d10 + 8) piercing damage. 

Tail. Melee Weapon Attack: +10 to hit, reach 30 ft., 1 target. Hit: 17 (2d8 + 8) bludgeoning damage. On a hit, the creature is shoved 20 ft. away from the eel in a straight line. 

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Lightning Breath (Recharges 5-6). The eel exhales lightning in a 90-­foot line that is 5 feet wide. Each creature in that line must make a DC 19 Dexterity saving throw, taking 66 (12d10) lightning damage on a failed save, or half as much damage on a successful one.