Legendary Resistance (2/Day). If Argrost fails a saving throw, it can choose to succeed instead.
Spellcasting. Argrost is a 5th-level spellcaster. Its spellcasting ability is wisdom (spell save DC 15 +7 to hit with spell attacks). It has the following cleric spells prepared.
Cantrips (at will): Toll The Dead, Light, Mending, Thaumaturgy
1st level (4 slots): Bane, Shield of Faith, Command, Create or Destroy Water
2nd level (3 slots): Aid, Hold Person, Enhance Ability Spiritual Weapon, Lesser Restoration, Misty Step
3rd level (3 slots): Animate Dead, Clairvoyance, Stinking Cloud, Conjure animals
Quarterstaff. Melee Weapon Attack +2 to hit. 1d6 damage.
Argrost can take 3 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. Argrost regains spent legendary actions at the start of its turn
Drain. When one of Argrost's undead minions damage a creature he can spend two legendary action points to regain the damage dealt as HP.
Detect. Argrost makes a wisdom (Perception) check.