Carnage Maw. Each time the z-rex damages an organic creature with its bite attack, its bite gains a +1 bonus (stacking) to its necrotic damage. This bonus resets 1 hour, at which point the festering gore of its victims slaw off its teeth.

Stench. Any creature that starts its turn within 10 feet of the z-rex must succeed on a DC 14 Constitution saving throw or be poisoned until the start of its next turn. On a successful saving throw, the creature is immune to the z-rex's stench for 24 hours. 

Undead Fortitude. If damage reduced the z-rex 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 success, the x-rex drops to 1 hit point instead

Multiattack. The z-rex makes two attacks: one with its bite and one with its slam. It can't make both attacks against the same target. 

Bite. Melee Weapon Attack: 1d20+10 to hit, reach 10 ft., one target. Hit: 20 (2d12 + 7) piercing damage plus 13 (2d12) necrotic damage. If the target is a creature, it must make a DC 14 Constitution saving throw or become infected. An infected creature takes a -1 penalty to all rolls they make. They can remake their saving throw at the end of each of their turns. Each time a creature fails this save the penalty to their rolls increases by 1.

Slam. Meele Weapon Attack: 1d20+10 to hit, range 10 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage. If the target is a creature, it must make a DC 17 Strength saving throw or be either knocked prone, or pushed back 10 feet. The z-rex can choose to make this attack recklessly, giving it advantage. If it does this . the next attack against it also has advantage. 

You have none.

Opportunity Attack. The z-rex makes an attack if someone leaves his range.