Pet Likelyhood. This creature can be tammed more easily, you just need to display friendliness to it, if it likes you it will not attack you and if you act in a certain way this creature can become your pet. (dm determines what act in certain ways means).

Evolution. As soon as this creature meets the conditions it will be able to Evolve, this can be controlled by Watcher creature. This creature gains spell slots with each evolution level. Evolution spells cannot be removed unless they are pernamently traided for something else. The DM can give you spells to choose from, if you dont like them the creature will roll for wisdom save, if failed you can make the trade, DM once again gives you spells to choose from, You HAVE to choose a new spell.

Petrifying Gaze. If a creature starts its turn within 30 feet of the Wrissaithious and the two of them can see each other, the Wrissaithious can force the creature to make a DC 12 Constitution saving throw if the Wrissaithious isn't incapacitated. On a failed save, the creature magically begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.

A creature that isn't surprised can avert its eyes to avoid the saving throw at the start of its turn. If it does so, it can't see the Wrissaithious until the start of its next turn, when it can avert its eyes again.

If it looks at the basilisk in the meantime, it must immediately make the save. If the Wrissaithious sees its reflection within 30 feet of it in bright light, it mistakes itself for a rival and targets itself with its gaze.

Bite: +10 to hit, reach 5 ft., 1 target. Hit: 17 (2d10 + 6) piercing damage. 

Constrict. +10 to hit, reach 5 ft., one creature. Hit: 15 (2d8 + 6) bludgeoning damage, and the target is grappled (escape DC 16). Until this grapple ends, the creature is restrained, and the snake can't constrict another target.

Tail whip. +10 to hit, reach 10ft., one target. Hit: 19 (2d12+6) slashing damage.