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
Cantrip. Argrost casts a cantrip
Spellcasting (Costs 1 action per spell level). Argrost casts a spell
Detect. Argrost makes a wisdom (Perception) check.
On initiative count 20 (losing initiative ties), Argrost can take a lair action to cause one of the following magical effects, Argrost can't use the same effect two rounds in a row
1. Argrost rolls a d6 and divides it by two. Argrost regains one spell slot of that number or lower.
2. Argrost turns invisible.
3. Argrost summons two undead minions.