My homebrew party is coming to the big season finale encounter and I want it to be memorable, dangerous, and perhaps deadly (but not impossible). 

The "boss villain" of this season is an aquatic beholder who commands a fleet of pirates. The party will have to encounter it in its underwater lair, complete with merrow guards and eyestalks growing like anemones all throughout the submerged cave that can see (and cast rays) into pretty much every crevice of the cavern. I know the anti-magic field will be tough on their ability to breathe underwater, so they'll have to be creative and resourceful (they have already been successful in gathering info, so they know what they're going up against ahead of time). 

This community as a whole has way more creativity, smarts, and cruelty in it than I do in my own imagination, so I would love to hear any suggestions y'all have to make this an epic encounter. The characters will be 8th-9th level when this happens. Thanks!

The thing to remember about beholders is they are not actually insane. They just appear to be because they have so many plans going simultaneously. Make sure he has a plan not just for what to do in the fight but how to escape if the fight doesn't go his way. He has the advantage in this situation, it's his lair and if he knows trouble is coming then, he will have a way out. This gives you a chance to engage in a fun chase scene if you want as you force the players through the gauntlet of traps designed to hinder and harm. This is also probably not his first fight, beholders gather minions, yes, but it is through their strength that they gather and subjugate these minions. He gets to use 3 of his rays a turn, every time a player acts there is a chance that he could use his rays on them.

Some uses for rays that an experienced beholder may use (assuming he has the standard rays) 

Charm Ray: Do not send the charmed creature to attack its friends. Instead, use it to give the beholder an advantage. Help action is great for this. Other uses are human shields, pushing players prone, or just causing general disruption/distraction. 

Paralyzing Ray: Not a lot that can be done with this, it's a quick way to take someone out of a fight temporarily, but if the beholder is smart he will paralyze a target in such a way as its allies have to waste time/energy/actions saving/helping their paralyzed friend.

Fear Ray: Again not a lot that can be done with this in combat but having little alcoves feared beings can run into to cower is useful, especially if those alcoves are trapped. This is the advantage of knowing where the fight is going to happen and manipulating the battlefield to your advantage beforehand.

 Slow Ray: A lot like the paralyzing ray this is going to debuff a target, not a lot of ways to make this more useful other than to intentionally target a slowed player as they can not take reactions and only have a limited set of actions. Anything hit by this should be swarmed by minions if the beholder has them on hand and has trained them well. 

Enervation Ray: This is pure damage, even better it's a save or suck roll, target creatures that are already hurt or are soft targets. Like the slow ray, anything hit by this should be swarmed by minions.

Telekinetic Ray: So much fun to be had with this. Pull levers to activate traps, move targets around the battlefield into prepared hazards. Get creative, anything a person can do with their hands this can do out to a range of 30 feet.

Sleep Ray: Another debuff, takes a target out of the game for a while. See Paralyzing and Slow Ray for minion tactics. Remember an attack on a sleeping/incapacitated target is automatically a crit. 

Disintegration Ray: This is just...the best. Yes, you can use it to disintegrate a target, but other creative uses like destroying a opponents weapon/spell focus, or just disintegrating a 10ft cube of the ground under them so they suddenly fall into a 10ft deep pit are useful. Creating a pit under someone and then using the Telekinetic Ray to put a stone slab the beholder prepared beforehand over it is a good way to sequester an opponent tell you are ready to deal with it.

Death Ray: Pure damage, and prevents an incapacitated target from getting back in the fight.

On to Lair actions! Now lair actions take place at initiative count 20, aka at the start of every round. And you have to use each one before you can use any of them again. 

A 50-foot square area of ground within 120 feet of the beholder becomes slimy; that area is difficult terrain until initiative count 20 on the next round.: Area of denial is useful, even more so if the beholder is floating up out of reach of melee weapons or retreating. If you used disintegrate to make pits coat them in slime to make it even harder for opponents to escape. 

Walls within 120 feet of the beholder sprout grasping appendages until initiative count 20 on the round after next. Each creature of the beholder’s choice that starts its turn within 10 feet of such a wall must succeed on a DC 15 Dexterity saving throw or be grappled. Escaping requires a successful DC 15 Strength (Athletics) or Dexterity (Acrobatics) check. : This goes for two rounds, the round you use it, and the next round, then it wears off. This is great for helping with an escape by hindering the actions of the heroes. Or it can be used to keep them in place if they are stuck in hazards like acid or flaming oil or your run of the mill molds/slimes. 

An eye opens on a solid surface within 60 feet of the beholder. One random eye ray of the beholder shoots from that eye at a target of the beholder’s choice that it can see. The eye then closes and disappears.: See my rundown on eyes and their effects. 

Final note. Antimagic Cone, this is both a help and a hindrance, it should only be used on heroes if they have some sort of buff that is causing problems, otherwise, it will negate the beholders own magic. 

Yes, these tips and tactics will make the fight hard. In fact, there is a good chance it could kill your entire party. How you run this encounter is up to you, remember a beholder is Lawful if the party is losing it may give them a chance to become its minions if they sing its praises enough. If it sees it is wining it may stop to gloat for a bit. It may even use the fact that it has cowed the heroes to make its escape, leaving them alive as some part of another intrcate and wide spread master plan. Beholders always have a plan, and then a back up plan, and then a back up backup plan, plan a though z and twice around again. Players coming in to this should be ready to counter the eye effects and what ever tricks they may run into. 

Good Luck, Have Fun. 

Eye of deep. (the name of beholders that live underwater)

Xanathar, the name of a beholder that loves baths.

I must confess to some curiosity about how this encounter went. 

Sageis, thank you SO much for all the thoughts on the beholder fight. You brought up some points I hadn't considered (especially about using the terrain and traps to its advantage). 

That encounter won't be until later this month, but I wanted to plan ahead. I'll definitely let y'all know how it goes. Next session they'll have to sneak/fight/figure out a way past the pirates guarding the entrance and free the prisoners before facing the beholder underwater. If they play their cards right, they could find some more information and helpful items. 

We held the session today. It was as epic as I'd hoped for-- a truly scary, one-of-a-kind, and challenging encounter but the PCs were victorious in the end. 

The underwater caverns leading to the aquatic beholder's lair were lined with wormlike appendages, each one tipped with an eye that had been cut out of the heads of living prisoners by sea hags and pirates who worked for the beholder (known as "the Kaititiro"). These acted like security cameras averting the party's attempts to stealth in, and allowed the kaititiro to fire rays at the party as they entered certain sections of the lair. Merrow henchmen, traps releasing clouds of poisonous algae, and chuul-like aberrations that blended into the walls of a cavern by having barnacles and shells growing on their backs as camouflage all soaked up PC resources on the way to the final lair.

As the players were mostly 8th level, I adapted some of the beholder's rays, lessened the damage of some of the more severe ones (but they were still deadly!), and reduced its AC and save DC by 1. The lair included some alcoves and a deep trench that enabled cover from the kaititiro's rays, but risked setting off traps like the chain net that the kaititiro could release through its telekinetic ray, trapping adventurers in the trench where they couldn't escape the chuuls and the kaititiro could keep them within its antimagic field if it chose to. 

The antimagic field was a huge problem for the PCs because it cancelled the effects of their water breathing ritual while the PCs were within it. Two PCs dropped to 0 hp while in the field, so not only were they making saves but they couldn't move away from the field and were also actively drowning. Escaping it was especially challenging for those without a swim speed or means of traveling quickly, since the kaititiro positioned itself so that they entered where the cone was widest. They were moving half-speed while drowning and getting assaulted by henchmen.

The PCs had to get creative, but were victorious in the end. The monk shadow stepped behind the kaititiro and got off a stunning strike, allowing him to reposition its antimagic field so that a healing word could revive the downed ranger and she could breathe water again. With the antimagic field under control, the wizard polymorphed the kaititiro into a mostly harmless jellyfish (using his portent to ensure a failed save) so the party could focus on the henchmen and environmental hazards without worrying about the rays or antimagic field. Then they ganged up on the kaititiro and came out on top, but with three of the five party members at single-digit hp. Basically the first two rounds had the table genuinely freaking out about a possible TPK, but by the fourth round things had started to turn around. 

This combat was immediately followed by a skill challenge as they had to complete a ritual to close a planar fissure to the elemental plane of water that the kaititiro had been guarding, which almost sucked two of the PCs into the plane forever once it was successfully completed, but they both rolled very high Str saves and avoided catastrophe.

Thanks for the suggestions! They definitely helped spur my imagination and use the battlefield to create a memorable fight.

Sounds like a battle well fought! I'm glad it came out well and hope it is as memorable for the players as it is for you.