Homebrew
The lich is an 18th-level spellcaster. Its spellcasting ability is Charisma/Intelligence (spell save DC 19, +11 to hit with spell attacks). The lich has the following warlock spells prepared: Cantrips (at will): mage hand, prestidigitation, ray of frost, eldritch blast 1st level (4 slots): detect magic, magic missile, shield, thunderwave, silvery barbs, absorb elements 2nd level (3 slots): acid arrow, detect thoughts, invisibility, mirror image 3rd level (3 slots): animate dead, counterspell, dispel magic, fireball, spirit shroud, scrying 4th level (3 slots): blight, dimension door 5th level (3 slots): cloudkill, scrying, Flame Strike 6th level (1 slot): disintegrate, globe of invulnerability 7th level (1 slot): finger of death, plane shift, Teleport 8th level (1 slot): dominate monster, power word stun 9th level (1 slot): power word kill
Legendary Resistance (3/Day). If the lich fails a saving throw, it can choose to succeed instead. Rejuvenation. If it has a phylactery, a destroyed lich gains a new body in 1d10 days, regaining all its hit points and becoming active again. The new body appears within 5 feet of the phylactery. Dark One's Blessing. When The Lich reduces a hostile creature to 0 hit points, it gains 25 Temporary Hit Points. Dark One's Own Luck. Onca a day, The Harbinger can call on his patron to alter fate in his favor. When he fails an ability check or a saving throw, he can use this feature to add a d10 to his roll. Fiendish Resilience. Thomas has resistance to Radiant Damage. Hurl Through Hell. Once a day, when he hits a creature with an attack, The Harbinger can use this feature to instantly transport the target through the lower planes. The creature disappears and hurtles through a nightmare landscape. At the end of Thomas' next turn, the target returns to the space it previously occupied, or the nearest unoccupied space. If the target is not a fiend, it takes 10d10 psychic damage as it reels from its horrific experience.
Paralyzing Smite. Melee Weapon Attack: +11 to hit, reach 5 ft., one creature. Hit: 10 (3d6) cold damage. The target must succeed on a DC 18 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
The lich can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The lich regains spent legendary actions at the start of its turn. Cantrip. The lich casts a cantrip. Paralyzing Smite (Costs 2 Actions). The lich uses its Paralyzing Smite. Frightening Gaze (Costs 2 Actions). The lich fixes its gaze on one creature it can see within 10 feet of it. The target must succeed on a DC 18 Wisdom saving throw against this magic or become frightened for 1 minute. The frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target’s saving throw is successful or the effect ends for it, the target is immune to the lich’s gaze for the next 24 hours. Disrupt Life (Costs 3 Actions). Each non-undead creature within 20 feet of the lich must make a DC 18 Constitution saving throw against this magic, taking 21 (6d6) necrotic damage on a failed save, or half as much damage on a successful one.
On initiative count 20 (losing initiative ties), the lich can take a lair action to cause one of the following magical effects; the lich can’t use the same effect two rounds in a row: The lich rolls a d8 and regains a spell slot of that level or lower. If it has no spent spell slots of that level or lower, nothing happens. The lich targets one creature it can see within 30 feet of it. A crackling cord of negative energy tethers the lich to the target. Whenever the lich takes damage, the target must make a DC 18 Constitution saving throw. On a failed save, the lich takes half the damage (rounded down), and the target takes the remaining damage. This tether lasts until initiative count 20 on the next round or until the lich or the target is no longer in the lich’s lair. The lich calls forth the spirits of creatures that died in its lair. These apparitions materialize and attack one creature that the lich can see within 60 feet of it. The target must succeed on a DC 18 Constitution saving throw, taking 52 (15d6) necrotic damage on a failed save, or half as much damage on a success. The apparitions then disappear.
Galen Abaddon has made the Celestial Spire itself his own Lair.