Here is a 5e background inspired by "The Outsider" by Lovecraft, some of Poe’s claustrophobic short stories, a bit of Dumas (The Count of Monte Cristo, The Man in the Iron Mask), Mervyn Peake, Horace Walpole, and this awesome post by Jack Shear. There might be some Fallout in it too. Or Room, although I haven’t watched it or read more than a few pages.
The name was quite difficult to decide upon: “Castle born” sounds like a traditional noble background, something equivalent to “silver spoon”; “Tomb child” sounds vampiric or undead; “Shadow spawn” sounds like something out of the Shadowfell, etc. Of course, you can use any of them if you prefer.
This background is quite close to Sage, with a bit of Hermit, Noble and a Gothic flavor. If you like the idea but dislike the rules I’ve written, use Sage instead.
There are many interesting ways you can use that. A character like Bane, born in a prison, is one possibility; but a D&D version of they guy who spent all of his life in his mom's basement is also viable. Also, Rapunzel, Edward Scissorhands, etc.. A post-apocalyptic survivor like the protagonist of the Fallout games allows you to play the outsider who knows nothing of the current archetype in a fresh way. An artificial being, or a Quasimodo character are some of the possibilities I haven't explored to avoid making it too long.
You were born and raised into the depths of a huge castle, dungeon or tunnel complex. You have lacked nothing - you slept in fine linen, studied in huge libraries, and there was always a friendly hand to feed you. Whose hand is that, you don’t quite know – maybe a servant of your ancient family, or some forgotten creature or undead. You have never seen much of the world outside the walls (or maybe not even the light of the sun), or talked to common people, although you know they do exist somewhere. When you finally got out, you felt alienated from everybody, but also curious and eager to explore.
Inside & out
Why were you kept inside? Who raised you? You can create your own story with your GM or roll in the table below. You might also choose to not know all details beforehand, and find them out during play. Likewise, think of how you got out (did you escaped, reach maturity and was left to fend for yourself, walked away when all servants disappeared, was expelled, rescued by other PC’s), unless the adventure starts while you’re still inside.
1. I was a bastard unwanted by my highborn parents, kept secret to avoid shame.
2. I was abandoned and raised by benevolent spirits.
3. My caretakers were overprotective to the point of insanity, although I didn’t realize that.
4. I was kidnapped by a maniac and protected by another prisoner.
5. I was part of a doomsday cult that tough the world had ended.
6. The usurpers took my titles, but wouldn’t dare to kill me.
7. My parents were imprisoned and killed by underground creatures that took pity on me.
8. I am a child of the undead or inhuman – I should never be living in the first place!
Skill Proficiencies: Arcana, History.
Languages: Two of your choice.
Equipment: fancy and peculiar clothes, a lantern, a necklace with two old portraits (your parents?), 25 gp.
Feature: Born in the Darkness
You are used to move in the darkness, and you can see in dim light within 30 feet of you as if were bright light. You also have an instinctive sense that tells you of the existence of any secret doors or passageways within most buildings, as well as the purpose of most rooms.
Variant feature: Forgotten lore
You have acquired a huge volume of curious forgotten lore. Even when you do not know a particular piece of information, you often know some ancient and relevant trivia related to it. You might not know a particular noble, for examples, but would have heard about his ancestors or past landowners.
Darkness and isolation leave permanent marks on the dungeonborn. Whether curious about their own origins or eager to get away from them, they are still quite alien and ignorant of common people’s customs and manners. They can be shy, strange and socially inept, or adorable in their exotic naiveté.
1. I lack social skills and often commit innocent but embarrassing faux pas.
2. I am shy and get embarrassed when dealing with extroverted people.
3. I am eager to make new friends and treat everyone I find as a potential ally.
4. I am extremely curious about the outside world and marvel at the simplest things.
5. I am dismissive of people with little or no book learning.
6. I constantly quote from forgotten books that no one else has read, expecting to be immediately understood.
7. I do not know how to deal with money.
8. I am incredibly patient and think long–term in most of my actions.
1. Freedom. No one should be forced to live in a prison of any kind (Chaotic).
2. Knowledge. I must learn as much as I can from this huge, strange world (Neutral).
3. People. Most people are inherently good and should be treated as such (Good).
4. Traditions. I must keep the traditions of my family, no matter how the world sees them (Lawful).
5. Power. I will show this degenerate, ignorant people that they should do as I say (Evil).
6. Self-knowledge. I must find where I came from and what I can become (any).
1. I am the last heir to my family and I must protect their good name.
2. These people are my first friends and I must treasure them forever.
3. I is my duty to maintain my lands and castle.
4. This picture is the only remainder of my ancestors, I shall never lose it.
5. I will take revenge against the ones who imprisoned me.
6. I will return to the castle one day, and make it a better place.
1. I look down on the ignorant masses.
2. I cannot trust anyone.
3. I was imprisoned by my own fault and want to keep it a secret.
4. I am vengeful and cannot forgive an insult.
5. I am oblivious to other peoples’ feelings.
6. I believe everyone should pay attention to my needs.