Fey Ancestry : Magic can't put Feal-Thas to sleep.

Trance : Feal-Thas doesn't need to sleep. Instead, he meditates deeply, remaining semiconscious, for 4 hours a day. After resting in this way, he gains the same benefit that a human does from 8 hours of sleep.

Spellcasting. Feal-Thas is a spellcaster that can cast spells up to the level 6. Its spellcasting ability is Charisma (spell save DC 19, +9 to hit with spell attacks). Feal-Thas can cast the following spells that were given to him by Tiamat : Armor of Agathys, Breath Weapon, Cone of cold, Ice storm, Investiture of ice.

Additionaly, he can also cast : Detect magic, Levitate, Light, Mage hand, Shillelagh.

Staff +2 : Weapon Attack: + 7 to hit, reach 5 ft., one target. Hit: 1d8 + 7 bludgeoning damage

Breath Weapon : As an action, Feal-Thas can exhale destructive energy. When he uses his breath weapon, each creature in the area of the exhalation must make a Dexterity saving throw. The DC for this saving throw equals 14. A creature takes 8d6 poison damage on a failed save, and half as much damage on a successful one. Distance : 5 ft. 30 cone. Each turn, roll a d6. On a 4, 5 or 6, Feal-Thas gains the ability to recast this spell.

Arcane gate (2/long rest) (concentration) : Feal-Thas creates linked teleportation portals that remain open for the duration. Feal-Thas chooses two points on the ground that he can see, one point within 10 feet of him and one point within 500 feet of him. A circular portal, 10 feet in diameter, opens over each point. The portals are two-dimensional glowing rings filled with mist, hovering inches from the ground and perpendicular to it at the points you choose. A ring is visible only from his side, which is the side that functions as a portal. Any creature or object entering the portal exits from the other portal as if the two were adjacent to each other; passing through a portal from the nonportal side has no effect. The mist that fills each portal is opaque and blocks vision through it. On his turn, he can rotate the rings as a bonus action so that the active side faces in a different direction.

Armor of Agathys (3/short rest) (1 hour) : As an action, a protective magical force surrounds Feal-Thas, manifesting as a spectral frost that covers him and his gear. He gains 30 temporary hit points for the duration. If a creature hits him with a melee attack while he has these hit points, the creature takes 20 cold damage.

Cone of cold (3/long rest) : As an action, a blast of cold air erupts from his hands. Each creature in a 60-foot cone must make a Constitution saving throw. A creature takes 8d8 cold damage on a failed save, or half as much damage on a successful one. A creature killed by this spell becomes a frozen statue until it thaws.

Ice storm (1/short rest) : As an action, a hail of rock-hard ice pounds to the ground in a 20-foot-radius, 40-foot-high cylinder centered on a point within 300 feet. Each creature in the cylinder must make a Dexterity saving throw. A creature takes 4d8 bludgeoning damage and 8d6 cold damage on a failed save, or half as much damage on a successful one. Hailstones turn the storm's area of effect into difficult terrain until the end of Feal-Thas's next turn. 

Investiture of ice (1/long rest) (concentration) : As an action and until the spell ends, ice rimes Feal-Thas's body, and he gains the following benefits:

• He is immune to cold damage and has resistance to fire damage.
• He can move across difficult terrain created by ice or snow without spending extra movement.
• The ground in a 10-foot radius around him is icy and is difficult terrain for creatures other than him. The radius moves with you.
• He can use his action to create a 15-foot cone of freezing wind extending from his outstretched hand in a direction he chooses. Each creature in the cone must make a Constitution saving throw. A creature takes 6d6 cold damage on a failed save, or half as much damage on a successful one. A creature that fails its save against this effect has its speed halved until the start of your next turn.

Conjure elemental (1/long rest) : Feal-Thas calls forth an air or water elemental servant that fills a 10-foot cube within 90 feet. The elemental disappears when it drops to 0 hit points or when the spell ends. The elemental is friendly to Feal-Thas and his companions for the duration. Elemental plays at the same turn as Feal-Thas. It obeys any verbal commands that he issues to it. If he doesn't issue any commands to the elemental, it defends itself from hostile creatures but otherwise takes no actions.

Legendary Resistance (1/Day) : If Feal-Thas fails a saving throw, he can choose to succeed instead

Feal-Thas can take 2 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Feal-Thas regains spent legendary actions at the start of its turn.

Frost finger : Feal-Thas points his finger at a creature within 60 feet of her. That creature must succeed a Constitution saving throw (DC 23) or take 5d6 cold damage and have a disadvantage on their next attack.