The Rotting Queen

So-named because pieces keep falling off her miniature

There she is, the Rotting Queen. Half-shadow elf, half-spider, ruling forgotten halls beneath Broceliande.

The majority of the ancient and terrified community of grey elves she has enslaved have not seen the sun in 300 years, they do nothing but bemoan their fate, obey her obscene commands, and occasionally notice body parts keep falling off her.

The actual spider-elves and Unseelie Fae consider her a grotesque, she makes them uneasy (do you know how fucked up you have to be to make the Unseelie Fae uneasy?), but they do not know her secret.

This is what "she" really looks like. The actual Queen died centuries ago, a small squirming daemonic parasite that inhabits her left eyes has been puppeting her husk since at least the end of the Malachite Era. It has absorbed her personality, believes itself to be the Queen, and will squirm off to find a new host if given half a chance.

It eats magic.

Her lonely realm looks like a Piranesi drawing lit by Richard Corben:

Traps and creatures down there will be:

40% Necromantic

40% Spider-themed

20% Weird magic like your arms slithers off and grows a tail and hates you.

She has a panther, and will you throw you in a pit with him if you become inconvenient.

Her halls are patrolled by Danse Macabre skeletons, who ensure, peace, order, obedience, and a constant eerie background music.

The halls are lit and irradiated by emanations from the Eldritch Fountain...

...which must be periodically fed the blood of high-level magic-users...

So expeditions outside the catacombs are regrettably necessary. For these purposes, the Queen relies on a cadre of hunter-necromancers...

...to kidnap sorcerers and bring them back to her realm.

The actual process of extracting the magic from a wizard takes hours and involves a lot of hooks and chains and is completely excruciating, though one in ten wizards actually like it. They die happily in the octile embrace of the Rotting Queen.

The good news is they have to keep the wizard alive to do it, so there's usually just enough time to:

1. Realize you lost your wizard and they aren't just dicking around in the library
2. Find out from locals that they saw your pointy-hat friend being carried off by what looked from a distance like maybe some drunk friends in a reverse-Weekend-At-Bernies situation 
3. Go oh, wait, zombies
4. Find out the nearest source of zombies is the Rotting Queen's catacombs
5. Mount a daring rescue

Now here's the kicker: since only the blood of high-level magic-users is useful to the Rotting Queen, The Rotting Queen is eager to make herself useful to low-level magic-users. She wants you to level up.

If you're out there with your party members, killing hill giants and robbing crypts and making a name for yourself, buying everybody drinks at the tavern with thousand year old coins, you may eventually get a knock on the door.

And if you follow the hooded and black-clad figure you find standing there, they will lead you down and down again to the Rotting Queen, who will be happy to help you--as the butcher is happy to see the spring lamb fed.

She will assume a pleasing form.

The tome you seek? She knows where it is.

The creature you must fight? She knows its weakness.

It's all going to be fine.