An exact error, as well as the modpacks (or mods, if it's custom) in use when the crash happens are necessary before anyone can help you. I probably can't help you anyway, but it will make it easier for someone who can to help you out.
Hey everyone who cares to read this, welcome! Anyway, it's been years since I played Minecraft, and I've just gotten back into it. Currently the map I'm in, with where I'm at, has straight up inspired me to build a Secret World themed pack. Unfortunately, I have little grasp on appropriate mods that would be useful toward this goal. I have some in mind, like Immersive Engineering, Malisis doors, Storage Drawers, Tinker's Construct, and Sync, but I'm in need of a lot more to get the atmosphere right.
I'm using 1.12.2, since it seems to have the broadest support as far as I can tell... and the seed that inspired me is on that version (and I'm not sure if newer versions still change the seed generation). Currently I'm only going to build this for myself, and if it's something I think I can share, I'll contact the mod authors and all that jazz.
Things I'm looking for:
Technology-wise, I need a bit of everything from pre-civilization-esque Magic, to Ancient Egyptian and Babylonian type magic and artifacts, to Greek\Roman\medieval contraptions, to industrial era, to modern technology, to futuristic labs.
I need to build a World Tree dimension, like Agartha from the games.
Creatures: I need monsters. Lots of mythical looking monsters. And definitely some Lovecraftian themed horrors.
Looks-wise... about everything you'd expect from the above.
I don't mind compromising some things, as I know I won't get everything; I'm even planning to adjust the mods as I need them to cut out duplicates and unnecessary things. I'm just looking for suggestions that I can look through. If you don't mind, could you please add a one or two word descriptor to any links you provide or suggestions you make? Thank you so much for anyone who reads this and drops a suggestion.
Worst case scenario: I do some programming along the way. I don't mind too much.
Best case scenario: Someone has already done this (if so, please let me know).
And even if the mod has only one useful thing, let me know (and please tell me what that thing is so I don't go insane looking for it) about it. Thank you for at least reading this!