Buppido (AOM)

Magic Resistance. The derro has advantage on saving throws against spells and other magical effects.

Spellcasting. The derro savant is a 5th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 12, +4 to hit with spell attacks). The derro knows the following sorcerer spells:

Cantrips (at will): acid splash, mage hand, message, prestidigitation, ray of frost

1st level (4 slots): burning hands, chromatic orb, sleep

2nd level (3 slots): invisibility, spider climb

3rd level (2 slots): lightning bolt

Sunlight Sensitivity. While in sunlight, the derro savant has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

No Melee Weapons. Only uses magic to attack.

AOM #1: Worshipping Throng. Trigger: Combat starting. As a reaction during his first move, Buppido gestures expansively, and cackles, "Meet my precious worshipers..." The bones on the floor begin to rattle, and come together, forming mis-matched skeletons  (Use each with two heads. There is one for each party member.) They take the remaining time in the round to form, and then act in subsequent rounds on Initiative round 18.  Use Skeletal Warriors to attack strong PCs and Skeletons to attack weak ones.

AOM #2: I AM Your GOD! I Am EVERYWHERE!  Trigger: 50% HP. When he reaches 50% HP, as an immediate reaction, Buppido cackles, "You must surrender to me. I am your God!" He waves his arms casting a spell. 1/2 of the remaining skeletal worshippers explode with a nimbus of power that surges into Buppido and powers the spell. He cases Greater Invisibility. (Detectable by normal means, does not end when he attacks, but only at his death, or at AOM#3) You can hear his voice echoing all around the cavern, singing a sing-song worship song to himself, and encouraging the party members to submit to him. The echoes prevent this sound from being used to locate him as long as he remains in the cavern.

AOM #3: I WILL ONLY BE STRONGER Trigger: 10% HP. When he reaches 10% HP, he will begin his next action by appearing in an advantageous position withing 30' of his last known location. (This does not count as his movement.) He is wounded badly and profusely bleeding. He cackles with insane certainty, "This is all...  a part of my plan.  You are already serving me! I cannot be killed.... Even if you ruin this fleshly body. I am death. I cannot be killed!"  He charges the nearest party member up to his full movement, casting Burning Hands. As per spell, all targets in 15' cone save DEX DC 14. Failures take 15 HP fire dam. Saves take 7 fire HP. If the target that he charged is within 5', that target takes double damage.