cocytus

 Ice Walk. Cocytus can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost him extra movement

 Magic weapons. Cocytus's weapon attacks are magical.

Action Surge(2/day). On cocytus turn, he can take one additional action

Martial advantage. Once per turn, Cocytus] can deal an extra 35 [10D6] damage to a creature it hits with a weapon attack if that creature is within 5 ft. of an ally of cocytus that isn't incapacitated

Legendary Resistance (3/Day). If cocytus fails a saving throw, he can choose to succeed instead

Innate Spellcasting. cocytus's innate spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). Cocytus can innately cast the following spells requiring no material components:

5/day each: frost bite, ray of frost, wall of ice(work just like wall of stone)

3/day each: ice knife ( cast using a 5th level spell slot), sleet storm, steel wind strike

1/day each: ice storm( cast using a 7th level spell slot), tenser's transformation

Multiattack. Cocytus can makes five attacks: one with his halberd, one with his katana, one with his flail, one with his battleaxe and one with his tail

decapitation fang halberd. Melee Weapon Attack: +19 to hit, reach 5 ft., one target hit: 49 (7d10+10) slashing damage

god slaying emperor blade. Melee Weapon Attack: +19 to hit reach 5 ft., one target hit: 51 (9d8+10) slashing damage

flail. melee Weapon Attack: +19 to hit reach 5 ft., one target hit: 28 (4d8+10) bludgeoning damage

battleaxe. melee Weapon Attack: +19 to hit reach 5 ft., one target hit: 33 (5d8+10) slashing damage

Tail. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage

Cold Breath (Recharge 5–6). Cocytus exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 23 Constitution saving throw, taking 72 (16d8) cold damage on a failed save, or half as much damage on a successful one

Cocytus can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn

Tail Attack. Cocytus makes a tail attack

Cast a spell (costs 2 actions). Cocytus can cast a spell

Frightening Gaze (Costs 2 Actions). Cocytus fixes its gaze on one creature he can see within 10 feet of it. The target must succeed on a DC 20 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 the lich's gaze for the next 24 hours