Traitorous. At the start of his turn, Addar does one of two things: stay loyal or turn traitor. If he stays loyal, he fights the side he was fighting before. If he turns traitor, he switches sides for the turn and fights the side he was allied with before. He only turns traitor if it benefits him in some way. If Lucifer Krell, monarch of undeath or a bone prince is fighting, nezerak is loyal to the side fighting against those creatures. 

Magical weapons. Addar's weapon attacks are magical.

Spellcasting. Addar is a 12th-level spellcaster. His spellcasting ability is intelligence (spell save DC 16, +9 to hit with spell attacks). A'xal has the following warlock spells prepared:
Cantrips (at will): eldritch blast, true strike, poison spray, mage hand
3rd level (3 slots): fear, counterspell, vampiric touch                                                                      4th level (2 slots): hallucinatory terrain, blight                                                                                  6th level (1 slot): create undead, eyebite

Multiattack. Addar makes 2 attacks with either of his weapons.

Raak'un, the moonstone blade. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 40 (4d10 + 5) piercing damage. 

Raak'un's bolt. Ranged Spell Attack: +7 to hit, range 30/60 ft., 1 target. Hit: 40 (4d10 + 5) force damage.

Shadows in another place.  In response to a creature opportunity attacking him, Addar temporarily teleports to the ethereal plane in a puff of dark mist before reappearing in the material plane again in the nearest safe position. 

Addar makes up to 3 legendary actions at the end of another creature's turn.

 Combination blow(Costs 1 Action). Addar swings Raak'un at one enemy within 5 feet of him. This hit does half damage. If the same target is hit a second time, the hit does normal damage. If that target is hit a third time, that hit does an extra 3d6 force damage on top of normal damage.