There have been a couple of fan attempts over the years to bring the worlds of Konami's Contra and Castlevania series together. Earlier this year, for instance, a hacker named I Love Pheromosa released Contravania: Dawn of Sorrow, a mod that let you take control of Contra ReBirth's Tsugu-Min inside the DS title Castlevania: Dawn of Sorrow and run-and-gun your way through a copy of Dracula's castle.
Now, it seems that another modder named ncoz has done something similar for 2008's Castlevania: Order of Ecclesia (as spotted by CD Romance on Twitter), again taking advantage of the modder LagoLunatic's DSVania Editor to combine these two very different worlds.
As the RHDN page states, in the mod, you'll be able to play as familiar characters such as Bill Rizer, Genbei Yagyu, Probotector, Newt Plissken, Brad Fang, and Sans from Undertale, though for the moment, you will need a fresh ROM for each of these.
The readme.txt gives the following description:
"An alternate timeline where Dracula's force has taken over the future of mankind, but wait...CONTRA is still alive! Contra has built their first ever time machine to go back to the past and is ready for some Dracula's ass-kicking duty!"
You can download the mod now from the RDHN page here. You'll then need to apply one of the six patches (one for each character) to your copy of the game using a trusted patching tool in order to get it to work on your DS emulator of choice.