Undertale Crossover Fanfic Novels & Stories

Honestly, AO3 is basically the holy grail for that kind of thing. Their tagging system is a lifesaver—you can just search "Undertale" and filter by "Crossover" or add another fandom tag like "My Hero Academia" or whatever you're into. The volume of stories there dwarfs everything else. I've found some truly bizarre but delightful crossovers, like a 'Deltarune' meets 'The Magnus Archives' fusion that had no right being as creepy-good as it was.

That said, don't sleep on FanFiction.net either. Their crossover category is a bit more old-school and less precisely tagged, but scrolling through the Undertale section there still yields some classics from when the game first blew up. The interface feels dated, but some writers never migrated. I stumbled upon a surprisingly heartfelt 'Undertale'/'To the Moon' mashup there last year that I haven't seen reposted anywhere else.

Tumblr can be a weird little side avenue, but it's hit or miss. A lot of writers will post snippets or links to their AO3 works, and sometimes you find threads or asks about crossover AU concepts that never got fully written but are fun to think about. It's more for the vibe and inspiration than a proper library, though.

Honestly, I don't seek out crossovers that often, but 'Dust and Determination' completely won me over. It blends the worlds of 'Undertale' and 'Dark Souls' in a way that's shockingly coherent. The core idea is that Frisk ends up in Lordran, and the story treats the 'Determination' mechanic as a form of the Darksign.

It's not just about fights; it's a philosophical exploration of cycles, mercy, and persistence that feels true to both games' bleak yet hopeful tones. The writing style is stark, matching 'Dark Souls'' environmental storytelling, while the characters from the Underground react to this new, decaying world with a mix of horror and fascination.

I keep going back to a specific scene where Sans encounters Siegmeyer of Catarina—the contrast between his laid-back sarcasm and Siegmeyer's jovial despair created a dialogue I didn't know I needed. The fic is complete, which is a huge plus.