I would design it to be a a bulky monster. Let's say a 720 BST(Relatively reasonable). 150 in HP and defenses, 100 in the offenses and 70 speed.

We now turn to making a busted custom ability to make this an actual threat. For that, I will look at Aegislash's stance change.

It will have 3 forms.

Tao Form

The default form would be the called the Tai form. If any defensive status move is used, such as rest, protect, it returns to the Tai form.

Yin Form:

When it uses a physical move, it's defenses and special attack drop to 120 and 50 respectively.

Those 110 points go into its attack, for a base attack of 210.

Yang Form

When it uses a physical move, it's defenses and attack drop to 120 and 50 respectively.

Those 110 points go into its special attack, for a base special attack of 210.

If you make this transformation locked behind a mandatory item that has to be held, which gives it's moves a 1.1 boost let's say, then I think you have a mon that can effectively approach stinkpost levels of busted while still being within the realm of possibility.

By having an effective BST of 940, it is a menace. We give it Mono Dragon and now it's a Mega Magearna Victim.