Undead Fortitude: If damage reduces the Undead Abomination to 0 Hit Points, it must make a Constitution saving throw with a DC of 5+the damage taken, unless the damage is radiant or from a critical hit. On a success, the Undead Abomination drops to 1 hit point instead.

Aversion of Fire: After this creature takes fire damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.

Multiattack. The Undead Abomination can use its Frightful Presence.  It then makes two slam attacks.

Frightful Presence. Each creature of the Abomination's choice that is within 60 feet of the Abomination and aware of it must succeed on a DC 15 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Abomination's Frightful Presence for the next 24 hours.

Slam. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 21 (4d6 + 7) bludgeoning damage.

Rock. Ranged Weapon Attack: +11 to hit, range 60/240 ft., one target. Hit: 23 (3d10 + 7) bludgeoning damage.

Necrotic Bolt (6). The Undead Abomination releases a necrotic bolt at a point it can see within 60 feet of it. Each creature within 10 feet of that point must make a DC 15 Constitution saving throw, taking 44 (10d8) necrotic damage on a failed save, or half as much damage on a successful one.