Disclaimer. The mechanics present in this article are still a draft. They are very potent and as such require editing and testing before recommended use at your tables.
When the adventurers first enter Za'Ul's Sanctum located in a sealed chamber atop the Bone Spire, he pays them no heed. However, should the adventurers dispatch Za'Ul's sentry Golems, he will put aside his current experiment to play host to his uninvited guests.
"What a mess!"
Za'Ul hisses inside your minds as you feel a sudden oppressive glare upon you. Then you see him. A man standing atop the North most pedestal surveying your group and the smashed pieces of the golem strewn about.
"It would seem I'll have to silence you all myself."
You might have Za'Ul enter the initiative after the third to last golem is defeated. This way, the players have the golems well in hand, but cannot spend their entire next round devoting all of their attention to Za'Ul.
He appears to the adventurers in his illusory form, an idealized version of his appearance in life. However, as Za'Ul uses abilities that require souls though, his form flickers to that of his true lich appearance.
Za'Ul employs the following tactics to outwit, outmaneuver, and outmatch his adversaries...
Za'Ul enters the combat quite annoyed at the intrusion of the players. While he uses his turns efficiently, they don't have his respect and don't warrant risking damage to his home. He tries to pick them off cleanly in the first round.
Za'Ul likes to systematically remove enemy creatures from the fight, like taking pieces from a chess board. He likes to focus them down one by one, dismantling the enemy. He is "relentless." Now that he realizes that the players may be a formidable threat, he begins to act more meticulously.
Soulless. A creature with the Soulless condition has disadvantage on all saving throws. Additionally, a soulless creature exposed to direct sunlight takes 5d6 damage at the start of their turn as they are reduced to a pile of smoldering ash. A creature may end the soulless condition by reducing Za'Ul, the Lich King to zero hit points, thereby freeing all souls captured by him. A soul that has been placed in the Vault of Souls can only be reclaimed by destroying the vault.
Za'Ul casts his spells at will as the situation calls for. He resorts to more and more destructive magics as he becomes more desperate to kill the characters. As he begins to run lower on souls for regenerative use and casting he might start to fling fireballs more freely with little care for the books and other valuable items in his sanctum.
Any characters that attempt to meddle with one of his treasures attracts Za'Ul's immediate attention and he swiftly deals with them by casting any of banishment, dominate person, feeblemind, power word stun, etc...