My group is coming to the end of our 4 year campaign, and I was hoping to get some ideas to help nail the ending. I had a final boss fight location in mind, with interesting mechanics, but that has been changed slightly by the party's actions. Now, the final fight will be against the Empyrean BBEG and some of its allies, during an attack on a city the party are trying to defend. 

I have two things that I am still working on. First is to buff the Empyrean a bit. The party will be level 20, with powerful magical items, and the basic Empyrean stat block I feel is a little lacking. If anyone has ran an Empyrean in the past or has any ideas the advice would be really helpful. One thing I'm considering is just giving him 2 more legendary actions each turn to help damage output etc.

The second issue is making the final fight interesting with the environment and other aspects of the fight than just the monsters. My original plan had a multi leveled fight caused by the earthquake spell for example. The party will most likely be defending within the city when the fight him. Any interesting ideas for terrain or monster interactions to add some epicness to this final fight would be great.

Thanks in advance for any help, I just want to see if I can find some inspiration to help this fight be a worthy finish to this campaign

Maybe you could add more at stake and give the empyrean  the ability to cast Wish, or buff the hammer slam legendary action to deal thunder damage, or call apon the powers of heaven to roll over the parry and smite them with its radiance. If you have any city figures you could also incorporate them into the fight, add in magical cannons or fortifications to the city. Course these are merely suggestions, take what you will. Ooh also add music, it really elevates a boss battle.

Triple its HP - Double the number of Legendary Saves it has - and make Solar Blast have 120' range and give him the ability to cast it after every players turn as a Legendary Skill.  increase the range on Solar flare to 90' and make the duration 1 round instead of 1 minute. 

I am assuming at level 20 all your players can fly pretty much at will - so by all means make the fight 3 dimensional that is always fun. 

That should make it more interesting lol 

Empyreans are not my favorite way to run god like entities. The empyrean stat block isn't really much to write home about, it's basically a giant, it hits with a big stick and throws rocks. Elemental rocks at a very long range but still rocks. I also don't particularly like the art It doesn't really radiate the kind of power I would like for it's station.  

 I prefer the multi stat block strategy seen in call of the nether deep and rime of the frost maiden.  

As for how to run empyrean there are a few things to note. 

• The empyrean has a 50ft speed which means without a speed boost it can simply kite players and use it's ranged attack. That is a very boring default win so try to avoid that scenario. 
• The empyrean can fly and swim. If players cannot they may have trouble following them around. Again combined with their ranged attack this is a boring default win. Avoid that scenario. 
• The Empyrean range attack is an astonishing 600ft. This can provide some unique opportunities for a sniper like encounter but if the players can't get  to them in a reasonable amount of time and have no cover from attacks then they will be whittled down little interaction.
• earthquake is a powerful effect but does nothing to flying creatures. It also doesn't work if you want the child of a storm god or something like that. You can replace it with something like storm of vengeance, reverse gravity control water or control winds  if it doesn't suit the scenario.  Basically any spell with a wide are that changes the terrain will get a similar effect.  You can also combine effects, this is a level 20 creature you can go  pretty crazy.
• The 1/day spells are " show stoppers". To make the empyrean interesting you really have to do something special with them. For example there may be lots of skirmish's going on and then the empyrean appears and they destroy them all with fire storm or maybe the crash into the earth causing an earthquake. As these are god like divine effects and empyrean aren't really spell casters so I'd consider treating them like powers as opposed to spells which means no counter spells.  It just isn't much fun if the 2 interesting things they get to do in an encounter gets cancelled 
• The stunned condition makes players automatically file dexterity and strength saving throws.  Weaker enemies with saves the players normally wouldn't fail can become brutal with this effect. They can follow up with own of their own spells or you can have enemies which call for dex or str saving throws. For example if  you  fight a empyrean in the middle of an arena surrounded by chanting priest if they get stunned then they could be facing an auto hit sacred flame from each priest. Allot of enemies like snakes, plants ect... have grapple attacks to take advantage of that too
   

How I would run it. 

1. I might start them about 300ft in the air with a bunch of weak minions fighting. They use bolt to strike  at key targets.  There may be player allied guards or alike that are also fighting. If the players are taking pot shots from cover they use earthquake to drive them out.  
2. One the players are detected they start descending. This takes 3 rounds with them using dash and their 3 legendary actions to attack with bolt. During this time they'd be very vulnerable and this is basically a dramatic entrance so I might give them some extra protection something like when they take damage instead they kill allies and use their health to reduce the damage. If you wanted you could also have the allies explode in divine flame.  I'd probably also give the area the effects of control winds down draft not effecting the empyrean to make the players flying up harder and give protection ranged attacks. If players do fly up getting whacked and stunned in that effect would be pretty devastating.  I think a slow descent, oppressive down draft and forced sacrifice of allies gives a good vibe of morally questionable divine being .
3. Once they land they use fire storm to destroy any remaining allies of the party before switching to melee.  To make it a bit more interesting I'd go either speed or strength. For speed I'd give them the effects of haste ( not including an extra attack) so they can move really quickly.I'd also let them move at half speed which with haste would be 50 ft as part of the attack legendary actions. For strength I'd make strikes have the effect of thunderwave aka aoe damage + knock back. If you use a strength save version a player who fails the save on stun automatically gets pushed 
4. When their health gets low if they haven't used it then use earthquake