The room, carpeted in bones and pools of coagulating blood and draped with sticky webbing reaching down from the vaulted ceilings, was the lair of a bebilith. The party's arrival was not silent, and thus the demonic spider noticed them immediately, scurrying out of the enormous pit in the eastern half of the room.
A bebilith is CR 10; I was using a slightly-modified version that came as part of this product on the DM's Guild. Alone it would be at a grave disadvantage despite the CR edge it has on the party -- action economy is king in 5E, after all. Thus, it was accompanied by varying sizes of giant spiders living in the ceiling, which descended a round later and continued to descend throughout the battle (totaling 12, ranging between CR 1 for the small ones and CR 2 for the large ones). Dynamic lighting in Roll20 helped mask some of the elements I needed to control in the shadows -- have I mentioned how much I love Roll20?
At this point the party was about about two-thirds strength. History demonstrated that the cleric can pull out some significant clutch heals and that the pal/sorc in particular can land some pretty disgusting alpha strikes, so I knew not to put all my spider eggs in one basket. The bebilith was the biggest threat, but poison and minor damage from the smaller spiders could not be ignored.
The fight unfolded pretty much the way I expected it to. Quickened blur is the go-to defense of the plate-wearing paladin/sorcerer, though his hit points are comparatively low -- the defense strategy is to avoid being hit in the first place. The battle master engaged the bebilith with Goading Attack to keep its attention on her while things were handled accordingly by the others. The ranger kept pressure up on the smaller spiders and made good use of Horde Breaker (when circumstances allowed) as well as her Sharpshooter feat; spider ACs aren't terribly high.
If you've not had the opportunity to play or DM for a shadow monk, they have downright crazy mobility with the teleporting they can do. Though it requires dim lighting, that is rarely a problem; she uses ki to grant darkvision to the party members that lack it and the group frequently moves in darkness to maintain the element of surprise on certain foes. Torchless Darkest Dungeon runs, anyone? In this case though, a torch was lit due to needs prior in the dungeon, making opening movements tricky.
The cleric previously made good use of the banishment spell to clear the board of dangerous opponents while the weaker ones were swept up, though the bebilith made its save against her attempt in this encounter -- this was for the best, since the bebilith long ago devoured the magic key that unlocked the marilith's prison and kept it in its gullet. Losing it would have meant having to open the prison a completely different way.
Web clumps were some terrain hazards, though they never came into play -- the group coincidentally never stepped into them, which is okay. The bone heaps were difficult terrain, though it also never really hindered the group.
In the end, the party was down to about one third of their resources and acquired a sizable sum of treasure from the bebilith's lair, in addition to the key they needed.
There are a few things I did wrong with this encounter, and in retrospect would have handled quite differently.
The spiders proved to be only a nuisance. They did not provide the desired output of harm to the party, though this was largely due to dice luck (as anyone will attest, you have those nights where you just can't hit to save your life) and the party remaining clustered. Thus, the spiders could not for long get through to the back rank to eat the tasty and squishy cleric and archer before they were splattered by either the paladin/sorcerer or the monk.
I should have played the bebilith more aggressively. At this point, some of the party members were still lacking magical weapons, and thus it held its ground very well in terms of absorbing damage. The battle master did her job very well in keeping its attention (inflicting disadvantage on some of its attacks on the others nearby by virtue of the Goading Attack early in the fight), but I should have focus fired a bit more to put the fear of the Abyss into the players.
Of course, powerful healing from the cleric was a factor. It's tough to find that sweet spot where I'm scaring the PCs, but not forcing the cleric to spend every action throughout a fight tossing cure wounds around just to keep people upright.
Overall though, the encounter was a success. The players felt well-rewarded and challenged, and I learned a bit more about their capabilities when the chips are down. Subsequent encounters would be based on the lessons I learned here, though now the party is level 13 and their power has grown by leaps and bounds -- 7th level magic alone can do things to turn the tide on a dime.