In a mini campaign I’ve been running, I’ve been hyping up the leader of this organization. He’s stayed mostly behind the scenes sending lesser leaders out to harry the party, but a major battle is coming up with him, and I want to flesh out a statblock.

He’s the leader of an organization called the Order of the Thirteenth Hour which is a group of fey assassins and leaders, that collect magic items, and summon powerful beings from the world’s history. Part of the Order’s strength and methods are freezing time so they can infiltrate and assassinate problem targets. I’ve used hold person, stunning strikes, and several other class features to do this with lesser enemies to show this. 

The party he’ll be facing is around 15th lv, and consists of monk (open hand), warlock (archfey), fighter (gunslinger), fighter (echo knight), wizard (divination). 

I think an AC of around 20 would be fine, as well as around 700-800 hp. 3-5 legendary resistances, and of course legendary actions. 

I’ve been thinking of throwing together some spells for his main attacks, but also would like any help on designing lair actions. If you also have a few fey abilities that would fit, please share! He would also of course have allies that aren’t terribly powerful, but it wouldn’t be him against the whole party.

Thanks for any help! (I know this is long)

My first thought is:

Not actually that physically powerful or tough.

....but he can use time stop at will.

This requires advance work to figure out a bunch of tricks he can use.

The other time-manipulator trope that comes to mind is self-cloning. Again, he's not a brick wall like you're envisioning, but there's five of him, and they work together perfectly.

Here’s a spellcaster BBE based around manipulating time and space Thai put together for someone else a little while back: (https://www.dndbeyond.com/monsters/3729938-pengriff-sanguine). Maybe it could be adapted to suit your needs.

You might want to consider an idea I borrowed from the Lancer RPG system: rather than giving him Legendary Actions, just give him multiple turns per round. If you want him to be able to go up against your party 1v5, give him five turns; have him alternate with the party so each time they go, he always goes next. You'd obviously need to adjust his per-turn damage output if you go this route, but it's a good way to overcome the usual action economy problems that boss fights present.

Also: it scares the crap out of your players as they realize it's happening. Highly recommend.

You can describe him casting a particularly powerful form of Haste on himself to explain why he can do this, and maybe you only introduce that once he's taken enough damage to go to his "second phase" or something.

Addendum: if you go this route, you'll also need to modify certain player abilities (Stunning Strike comes to mind) that suddenly become extremely powerful against this type of enemy. I would just give the boss the ability to repeat the save at the end of each of his turns against conditions that normally last a full round.

Quote from jl8e >>

My first thought is:

Not actually that physically powerful or tough.

....but he can use time stop at will.

This requires advance work to figure out a bunch of tricks he can use.

You could also just flavor other spells (steel wind strike comes to mind) and Legendary Actions as time stop-like abilities

I also suggest taking a look at the time dragons from Planescape. 

Give the creature the ability to eat spell slots.

Then it uses that spell power to enhance its abilities.

not a brick wall like your envisioning, but he's got a modified version of the teeth of dalev-nar