Amphibious. The zilla can breath air and water.

Magic Resistance. The zilla has advantage on saving throws against spells and other magical effects.

Asexual Reproduction. Every day the zilla is not being attacked it will spawn a zilla egg these eggs will hatch after 7 (1d6 + 1) days. The eggs have 15 (1d20 + 5) Health and a vulnerability to fire damage. If the eggs hatch a zilla baby emerges from the egg and fallows the zilla on its rampages.

Multiattack. The zilla makes three attacks, two with its bite and one with its tail.

Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit:  19 (2d10 + 8) slashing damage.

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit:  19 (2d10 + 8) piercing damage.

Tail. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit:  19 (2d10 + 8) bludgeoning damage.

Roar. The zilla emits a powerful roar. Each creature within 500 feet of the zilla and able to hear the roar must make a DC 15 constitution saving throw or be frightened till there next turn and deafened for the next hour.

Radioactive Fire. The zilla targets a creature it can see within 40 feet of it. The target must make a DC 17 constitution saving throw, taking 40 (8d10) fire damage or half as much on a successful one.  

The zilla 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 zilla regains spent legendary actions at the start of its turn.

Bite Attack. The zilla makes one bite attack.

Tail Attack. The zilla makes one Tail attack.

Radioactive Fire (Costs 2 Actions). The zilla makes one radioactive fire attack. It will be the same as the regular radioactive fire attack, but ranging 50 feet instead of forty.