Destructive Nature. Ad Mortem Rex has lived for so long that it is capable of destroying itself and reassembling itself somewhere else.
Spellcasting. The Ad Mortem Rex is a 20th-level spellcaster. Its spellcasting ability is 30 (spell save DC 20, +12 to hit with spell attacks). The Ad Mortem Rex has the following wizard spells prepared:
Cantrips (at will): fire bolt, ray of frost, poison spray, acid splash, shocking grasp
1st level (6 slots): ice knife, burning hands, floating disk, thunderwave
2nd level (6 slots): web, darkness, spider climb
3rd level (5 slots): fireball, lightning bolt, Antimagic field
4th level (5 slots): ice storm, wall of fire, necromancy bolt
5th level (4 slots): animate dead, cloudkill, wall of force
6th level (4 slots): necromancy bolt, circle of death, chain lightning, create undead. 7th level (3 slots): delayed blast fireball, forcecage, finger of death
8th level (3 slots): maze, power word stun, sunburst
9th level (2 slots): power word kill, meteor swarm, wish
Multiattack. Ad Mortem Rex makes two tail spike attacks and one humanoidslayer attack.
Humanoidslayer. Melee Weapon Attack: +10 to hit, reach 20 ft., 1 target. Hit: 60 (10d12 + 0) slashing damage, plus 30 (10d6) force damage.
Tail Spike. RangedWeapon Attack: +15 to hit, range 120/240 ft., 1 target. Hit: 100 (2d100 + 0) piercing damage, plus 12 (4d6) bludgeoning damage.
That would be very, very, challenging to fight.
I know, that’s why I made it.
It´s not really that powerful, if my party just uses our modified "Magic Missile" 3 times on ninth level... (1D4 + 9) Damage and 12 per spell. What I am trying to say is, that you did not really tried to make a powerful boss, but you just did the same WotC already did with the tarrasque. It´s not really a powerful being, but just a bunch of hitpoints with some cool abilities.