To put into perspective just how utterly, absurdly unreasonable Zacian is:
Let's take Mega Rayquaza. A Pokemon which caused the creation of a new meta because it was so strong that it had to be banned from the banlist, and which has been the sole occupant of that meta for two generations straight. A Pokemon which has been considered THE gold standard for over-the-top power creep since its inception.
Let's take away its special attack stat, which few sets used anyway. We'll also take away its priority, which was somewhat important but rarely essential -- it'll miss it, but whatever.
In return, we'll allow it to attack with the power of its Life Orb set without incurring recoil or any other disadvantage, and make that boost impervious to Knock Off. We'll give it loads more resistances and less weaknesses, such that only one Pokemon in the entire meta can hit it super-effectively with its STAB. Let's make that defensive power no longer conditional on maintaining a field effect that other Pokemon can overwrite or disable, and slightly improve its bulk.
We'll also remove its Stealth Rock weakness and turn it into a resistance, so it can switch out much more easily and be less reliant on team support. To this end, we'll make it immune to Toxic too.
We'll improve its speed by 33 points -- enough to outspeed every relevant Pokemon in the game unboosted, and outspeed neutral-natured scarfers up to base 95. This way, it can run Swords Dance more effectively, and still outspeed Marshadow which could otherwise ruin its day.
We'll give it far better STAB that hits the vast majority of relevant targets for at least neutral damage and plenty for super-effective. We'll give it coverage moves that hit every remaining target super-effectively within the other two moveslots, so it doesn't have to choose what it has trouble dealing with.
But wait, there's more! There's a new mechanic which allows any Pokemon to make itself twice as bulky! Let's give it a move that straight up negates that mechanic.
Finally, this will exist in a meta where the vast majority of previously top-tier threats to Mega Rayquaza no longer exist. There'll be a new steel-type threat with a gimmick which turns that threat into a physical wall, so one of our hero's coverage moves will hit it super-effectively while flat-out bypassing that gimmick. The other new threat will be less gimmick reliant, so we'll just make it outright immune to both of their STABs and completely stop that threat from boosting its attack power while our hero is on the field.
That's Zacian.