Apex of Death (Mythic Trait; Recharges after a Short or Long Rest). If Nethroi is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead it regains 200 hit points, mutates rolling on the table below.

Body and Soul. If a creature dies from damage dealt by Nethroi's bite, or from it's maximum hit points being reduced to 0 due to Nethroi's poison can't be revived by any means short of a wish spell. 

Keen Sight and Smell. Nethroi has advantage on Wisdom (Perception) checks that rely on sight or smell.

Legendary Resistance (3/Day). If Nethroi fails a saving throw, it can choose to succeed instead.

Life Stealer. If Nethroi deals damage to a creature with an attack, it recovers hit points equal to half the damage dealt.

Mutate. When Nethroi mutates, it releases a surge of unlife. It summons to its aid either 1 Tyrannosaurus Zombie or 3 Girallon Zombies.

Multiattack. Nethroi makes three attacks, one with its bite, two with its hooves.

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 19 (2d12 + 6) piercing damage and and 21 (3d10 + 5) necrotic damage. The target must succeed on a DC 22 Constitution saving throw or become poisoned. While poisoned in this way, the target can't regain hit points, and its maximum hit points is reduced by 16 (2d10 + 5) hit points at the start of each of its turns. The poisoned target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

The hit point reduction lasts until the target finishes a long rest. The target dies if its hit point maximum is reduced to 0.

Hooves. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 6) bludgeoning damage. 

Nethroi 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. Nethroi regains spent legendary actions at the start of its turn.

Claw Attack. Nethroi makes a claw attack.

Gaze of Death. Nethroi glares at a creature within 60 ft. The creature must succeed on a DC 20 Wisdom saving throw or become frightened of Nethroi for 1 min. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Nightmare Prowl. Nethroi moves up to half its speed. This movement doesn’t provoke opportunity attacks.