Got to run a multi-stage boss battle and it all went according to plan

The tower had a secret door near the entrance that led to the basement. The gang found this out after clearing all the upper levels. The goal is to rescue the Fire Spirit, trapped in this cage.

So Alekin hops down the stairs to the Fire Shrine and calls back up "Uh theres a LOTTA BONES in here"

Here's the first floor butted up against the basement. Makes for easier player movement even though there's still a staircase between the rooms. By now the party is interested in the skeletons and want to fight them.

I wish I'd taken more pictures of the battle itself, but most of the skeletons collapsed under the Death Druid's Undeath to Death spell.

With the skeletons gone, the Fire Spirit is freed from its cage, sending it on a rampage and burning everything in the room (I was having so much fun running this part I forgot to take any pictures)
They tried using water spells and ice spells to calm it or beat it up, but

None of it mattered when the fireplace in the back of the room collapsed suddenly.

A Disintegration ray shoots out from the Beholder's eye, destroying the Fire Spirit and exploding it into a cloud of ash and sparks. It extinguishes all sources of light in the room and begins to thrash the players with no mercy.

Behind the Beholder in the ritual circle was a mindflayer, watching but not participating in the fight.

This Beholder is special. This one is one they fought before. Instead of ensuring it was dead after shooting it a bunch of times, (hence the bullet hole) they cut off four of its eyestalks, (sold them) and then left it for dead in a mine that was about to explode.

For weeks it wandered the tunnels beneath the mountain, seeking refuge in the pit below the tower the gang just happened to be raiding. It would have seemed like a wild coincidence if it weren't for the mindflayer watching the whole show

Phase 1: -Skeleton Fight. There were 40 skeletons in the room. When combat begins, 4 skeletons nearest the Fire Spirit ignite and deal fire damage

Phase 2: -Fire Worm Fight. Fire Spirit is a 40 foot long lava worm. Radiant heat means everyone in the room takes D6 fire damage per round. Also it's got a mean breath weapon.

Phase 3: -Injured Beholder Fight. The Beholder crashes through the back wall, using its Disintegrate ray to explode the Fire Worm, destroying the original quest's goal and bringing back a bad guy that they said was "too easy" last time. Through the newly-opened hole in the wall the gang gets a glimpse of a Mindflayer, minding its own business apparently. It did steal one of the players' staffs (which is actually one of the Beholder's eyestalks and the activator to a doomsday weapon ...long story)

Phase 4: -Meat Bear Getaway. With the Beholder (finally) dead (for real this time), the Mindflayer goes ahead and summons two flesh corpses to block the doorways while it makes its escape into another dimension. As soon as the portal closes, the meat bears collapse into piles of flesh.