Arnoch, the keeper

Devil's sight. magical darkness doesn't impede Arnoch's darkvision.

Magic resistance. Arnoch has advantage on saving trows against spells.

Fire mastery. If Arnoch would inflict fire damage on an enemy creature it instead inflicts double that fire damage on that creature, and that creature caches fire. (This is not counted in these attacks/effects)

Innate spellcasting. Arnoch can innately cast the following spells, requiring no material components. His spellcasting modifier is charisma (+ 10 to hit with spell attacks, spell save DC 18):

At will: burning hands, command, hellish rebuke

3/day each: fire shield, haste.

Multiattack. Arnoch makes two swipe attacks. Or one slam attack.

Slam. melee weapon attack. +9 to hit. reach 5 ft. hit: 30 (5d10 + 5) bludgeoning damage. Any creature medium or smaller creature within 5 ft. of the point of impact must make a DC 14 dexterity saving throw or be knocked prone

Swipe. melee weapon attack. +7 to hit. reach 5 ft. hit. 12 (3d8) slashing damage. When Arnoch makes this attack it can make it against all creatures within a 5 * 15 ft. cube in front of him.

Arnoch has three legendary actions it can use them as described below. These actions cost no actions or bonus actions. Arnoch regains any expended legendary actions after it finishes a long rest.

Legendary save. Arnoch automatically passes a saving throw.

Molten earth (costs 1 action). Arnoch chooses a point that he can see within 30 ft. of him. The ground in a 20 ft. circle centered at that point starts to crackle with red-hot cracks. Any creature that finishes its turn in the area takes 6 (2d6 fire damage). The area lasts 1 minute.

Keeper's ward (costs 2 actions). Arnoch focuses inward on himself reminding himself of his duty as the keeper. Until his next turn he has immunity to any magical damage and effects from magic cannot affect him. If he is currently affected by any magical effects, they all end automatically.