Bone Wraith

Legendary Resistance (3/day). If the Bone Wraith fails a saving throw it can choose to succeed instead.

Magic Resistance. The Bone Wraith has advantage on saving throws against spells and magical effects.

Turn Immunity. The Bone Wraith is immune to effects that turn undead.

Multiple Heads. The Bone Wraith has advantage on Wisdom (Perception) checks and on saving throws against being blinded, deafened and stunned.

Magic Weapons. The Bone Arbiter's melee weapon attacks are magical.

Wielder of the Bone Arbiter. The Bone Arbiter is a +3 great axe that deals additional cold and necrotic damage to a target. Causing a debilitating weakness to the target on a failed saving throw.

Multiattack. The Bone Wraith makes two attacks with the Bone Arbiter.

Bone Arbiter. Melee Weapon Attack. 10ft reach; +15 to hit, one target. Hit: 3d12+10 slashing damage plus 3d6 cold damage and 3d6 necrotic damage. The target must also succeed on a DC15 Charisma saving throw. On a failed save the targets next saving throw or attack roll is made with disadvantage.

Bone Storm (Recharge 5 or 6). The Bone Wraith creates a ferocious whirlwind of bone and ice which spreads out from itself in a 60ft radius sphere. Each creature within this area must make a DC18 Dexterity saving throw, taking 8d6 piercing damage plus 8d6 cold damage on a failed save or half as much damage on a successful one.

The Bone Wraith 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 Bone Wraith regains spent legendary actions at the start of its turn.

Bone Spike Graveyard (1 action). Bone spikes erupt from the ground in a 20ft square centered at a point the Bone Wraith can see within 90ft. The area becomes difficult terrain until the Bone wraith is destroyed or until it dismisses the effect as an action. When a creature moves into or within the area, it takes 2d6 piercing damage for every 5ft it travels. 

Bone Arbiter (2 actions). The Bone Wraith makes one attack with the Bone Arbiter.

Coldflame (2 actions). A blast of icy flames stretches out in a 100ft long 10ft wide line from you in a direction you choose. Each creature in the line must make a DC18 Dexterity saving throw. On a failed save a creature takes 6d6 cold damage plus 6d6 fire damage and its movement speed is reduced by 10ft until the start of the Bone Wraith's next turn. On a successful save a creature takes half as much damage and its speed is not reduced.