I'm bored and I wanna talk about an idea I've been thinking a lot about lately.
Some things of note: the Marathon half is based on my AU, and the Castlevania half fudges a lot of things (this crossover isn't totally canon-compliant with either).
The basic setup is that a strange relic is uncovered on one of the UESC's larger colonies; said relic turns out to be Dracula's seal, which breaks and unleashes hell on the colony. The Rozinante happens to be nearby at the time, and Vince (aka the Security Officer) naturally heads down to try and help. He does well at first, but eventually hits a brick wall in the form of Death.
Death realizes that Vince is one of those three people whose souls he can't reap, on account of them constantly reincarnating (long story); he's about to do the job properly when a member of the Belmont clan jumps in to force a retreat. Unfortunately for Vince, Trista Belmont* is cold, standoffish, and rather insulted that the Rozies tried to do her job. And Vince has to work with her if he wants to prevent the colony from being completely overrun.
( * an old-ish OC of mine, not a canon Belmont)
Relevant art here and here. I can elaborate further on all this if anyone's interested.