Actions

Shield blash. Melee Weapon Attack: +6 to hit, reach 18 ft, one target. Hit: 9 (2d6 + 2) bludgeoning damage + DC -3 Strength.   Slash (longsword). Melee Weapon Attack: +3 to hit, reach 10 ft, one target. Hit: 3 (1d8 - 1) slashing damage, or 5 (1d12 - 1) slashing damage if used with two hands + 5 poison damage.       Block (Recharge 3–6). Blocks any damage under 3.

Bonus Actions

Undead healing (1/Short or Long Rest). Gain 8 (1d6 + 5) temporary hit points.

Reactions

Spark of death (1/Short or Long Rest). When Skeleton is reduced to 0 hit points but not killed outright, restore 9 hit points hit points and give 1 attack free turns.

Legendary Actions

Sacrifice. When Skeleton dies, he explodes in a burst of force and radiance. Each creature within 25 feet of him must make a DC 12 Dexterity saving throw, taking Ranged Spell Attack: +6 to hit, range 25/80 ft, ten targets. Hit: 23 (2d20 + 2) force damage plus 6 radiant damage damage on a failed save, or half as much damage on a successful one.

Lair Actions

Planar Portal - Necrotic. Rips in the Material Plane inside of the lair form portals to the Negative Energy Plane, allowing undead creatures to wander the lair.   All undead in the lair has advantage on saving throws against effects that turn until the start of the next round.   Supernatural shadows turn all bright light to dim light within the lair.   Until the start of the next round, all of the creature's minions become enraged. They gain advantage on melee weapon attacks, but melee attacks against them gain half the same benefit.   Magical energy swirls across the creature's skin, granting resistance to acid damage until the start of the next round.

a whinding lair leading back on its self, looks like a necromacer's lair, filled with necromacer like traps, and skeleton soldiers (and other types of skeletons, they also can aimbush you from hidden spots and the walls.  

Regional Effects

(if allyed) the whispers of cursed and undead knowadgle whisper in your head,allies of the monster have a 2+ advantage on all Wisdom checks within 5 miles of the lair. Known Terrain. within 8 miles of the lair,Difficult terrain is reduced in severity for the monster and its allies, decreasing the number of feet required to move 1 foot by 1. This means that most difficult terrain, which requires 2 feet of movement for 1 foot moved, is no longer difficult terrain. (if hostile)  Disease Ridden. Diseases are widespread throughout the area within 20 miles of the lair. Any creature that spends an hour or more traveling in this area will roll a (Constitution d12+4) if failed the creature is targeted by a disease of the GM's choice. Plants within 20 miles of the monster's lair grow, but are gray and devoid of any nutrition. No matter how many of them are eaten, they provide no sustenance and taste like ash and death.

Motivation/Compulsion

for revenge against the church and to take over the world

Usual Tactics

studying the movements of the person he is fighting and to dodge or block their attacks

Suggested Environments

underdark, mountain