Few things make me as happy as modders taking one game and stitching it to another. As Troy and Abed said, it only makes them more awesome.
Medieval 2: Total War is getting pretty old in the tooth, but modders are still working hard to find us some new ways to play it. The Elder Scrolls: Total War mod means you can now pit the various factions of Tamriel against each other in a war that can only be described as, well, total.
There are 20 factions to choose from, including The Kingdoms of Skyrim, Daggerfall, and Wayrest, the Clan of Blackmarsh and the Clan of Crowns, and the Great House of Hlaalu, Telvanni, and Dagoth, just to name a few. You can play as monsters too, like the hordes of Oblivion or an army of undead warriors. They're all custom skinned, so if you choose to side with Oblivion you'll really be marching around an army of Daedra.
There's a full map as well, based on The Elder Scrolls games, and appropriate custom banners will snap in the wind as you march your armies into battle.
The mod is not yet complete, but from my play session I'd say it's off to a good start. The campaign isn't finished yet, and there are some bugs, but I played a few custom battles and they worked just fine. It's also entirely in Russian, so in addition to the mod, which you can find a link for on this page, you may also need the English language patch. Even with it, some of the text still appears untranslated, though they're working on a better one.
Installing Medieval 2 mods isn't always easy, but there's a nice guide here on Reddit.