I am looking for a myth I can homebrew, or a preexisting monster in one of the handbooks that would kidnap a full grown adult. Most things I find mostly go after children and this person is in her late 20s to mid 30s (aka obviously not a child).

Basically a female NPC has been kidnapped from the party but she cant be killed right away by whatever takes her. I am looking for something that will kidnap her and take her to its lair (or somewhere similar) with future plans of some sort. It could be killing her in the future but I just need her to not die right off the bat when kidnapped if that makes sense. 

Please let me know any ideas! I am very comfortable with home-brewing and adjusting anything to my characters level so I really just need Ideas!

Vampires are ALL about abducting people and keeping them for long periods of time in their isolated lair, whether to slowly drain them of blood or to seduce them to their own dark path. 

Dragons also (more in folklore than in dnd lore) are famous for abducting people (mostly women) to add their beauty to their "hoard."

Otherwise it could be a person with a humanoid motive for wanting to abduct this specific person, i.e. ransom or political statement. 

Any of those doing it for you? 

The upper two are seriously on the right track, I didn't even think about dragons. 

Thank you!

Edit: if anyone has other suggestions i am still open. I have a lot of quests to do any anything helps 

I would suggest Doppelganger right up there with Vampires. They typically pursue and study their "prey" for long periods of time to accurately portray their look and persona, and that doesn't even have to be their sole motivation, and they are also often employed by other powers to do so. You could introduce a large open concept with any number of reasons why they would possibly keep quarry long-term.

Another really good Idea thank you!!!!!!

Depending on how dark your campaign is themed there are other reasons to keep females captive.

As for lighter mood stuff, Irish Fairy mythos involve kidnapping women so it might be worth while looking into that or doing something with the Fey. Could be as simple as a fairy circle disappearance, or have some higher tier arch fey implanting a seed into them during a ritual so they can grow a bit of their territory out into the mundane world.

Could also take some inspiration with movies in that regard such as one with a goblin king but instead of a baby, it could be for a wedding to secure legitimacy to a throne or a desire to sire an heir and just being smitten with their abductee. In that case could just fluff it to where they have a time limit for a celebration/ritual to cement it.

It could really be just about anything depending on motivation. A higher level Hobgoblin might have become obsessed with her like The Phantom of the Opera. An Orc shaman might have taken her as a (virgin?) sacrifice to it’s dark god and just needs to await the proper celestial convergence. Goblin slavers might have kidnapped her for future resale. 

Also, who knows why Beholders do anything?!? 

The kidnappers might even be working for a powerful third party and not even know that bigger bad guy’s enigmatic motivations spawning an even bigger quest.

Does your party have any enemies or groups that want to get back at them? In my game, the party was trying to catch a serial killer and, to goad them into actually finding him, the killer kidnapped one of their NPC friends and left a clue.

There have already been a lot of good suggestions here for monsters, but motive is just as powerful sometimes

You could go for a fey, they are famous for whisking people away as part of revels and then playing with them in the fey wild until they finally die or the fey grows bored of them. Or maybe a night hag that came and invaded her dreams and made her sleep walk to the night hags lair where the hag is holding her for some purpose? Or it could just be a human magic user that cast dominate person? Many many options

Least horny redditor

Grendel's Mother, from the poem Beowulf. Rather than overwhelmed with rage by the death of her son at the hands of an adventurer's patron or some other relation, Grendel's Mother kidnaps said patron during her plots and schemes, divising a cunning ruse to lure her hated foe to her. The patron knows some magics, or Grendel's Mother has the means to cast spells, either one is fine to have them communicate with the adventurer telling them to come with proof of life. Grendel's Mother, in all her patience, might even be able to feign a sense of fair play, perhaps offering a duel to the death or telling the adventurer they must give their life for their patron.

I don't know what monster you'd use, or if you'd want to designate your own stats and powers, but it's some inspiration and motivation for the character to get her way. I wish you luck!

Hades...

Here are a few suggestions I didn't see yet:

Mind flayers. These aberrations are all about enslaving, infesting, performing horrific experiments on, and/or eating humanoids. If she was kidnapped by these, the party might have the additional challenge of restoring her sanity or even blowing an illithid tadpole out of her brain. If you're dealing with high-level characters (3rd or 4th tier), this could be a fair challenge for them.

Wererats are a lower-level option. I can imagine these lycanthropes capturing someone to be added to their criminal group. Assuming they infest her with wererat lycanthropy as soon as she is kidnapped, she might be a little unwilling to leave, and perhaps resist the party's attempts to rescue her until she is targeted with Remove Curse or similar magic.

Devils! Suppose a devil captures this NPC and refuses to let her go until she signs her soul away in exchange for her freedom. Better yet, the devil could even approach the party and tell them that they must do it a favor (or even sacrifice a soul) if they ever want to see their beloved friend again. This idea works for characters of all levels, but adjust the CR of the devil type you choose accordingly. If you're looking to start a lengthier quest, you could even decide that she already signed her soul away for something by the time she reunites with the party, causing them to need to find some way to void the contract or kill the devil that made it on its own plane.

Genies of all sorts are known to take particularly attractive and/or talented mortals as captives and slaves.  Some giants, particularly fire giants and cloud giants might do so.  Krakens and aboleths take mortals for highly unsavory reasons.  A hag might kidnap a beautiful woman for a dark ritual.

Since it sounds like this is an intentional kidnapping the bad guy needs a reason to do this. Once you come up with a motive the choice of specific monster or villain is mostly flavor. Here are a few motives:

1. The captive has a memory that is the solution to a magical puzzle of some sort. Perhaps the captive has some strange runes tattooed on them or some significant birthmark.
2. The captive is an ingredient in a magical ritual. Perhaps a necromancer has figured out how to cheat death by transferring their personality into a new youthful body periodically. Think Rapinzel, or Tangled.
3.  The captive is valuable to a third party. Maybe their brother made an infernal bargain in exchange for “my sisters honor” or something similar. This has the amusing possibility of the party making a deal with the devil in exchange for the captive. 

Happy gaming!

There was a supplement to an older edition called Book of Vile Darkness. I hear it included a passage about a dragon kidnapping a princess in order to produce an half-dragon heir of royal blood. *shudder*

That kind of scenario definitely needs to be cleared with your players before you run it.  There are a lot of people who do not want to have that kind of content be part of their game.

Quote from 6thLyranGuard >>

That kind of scenario definitely needs to be cleared with your players before you run it.  There are a lot of people who do not want to have that kind of content be part of their game.

 Agreed.