Not beginning as a crossover, but will become. The end of the fourth season and the end of the second season. If you watch these shows you will know which season correspondant to show.

Powerful magic strands Damon in the Midlands with The One True Seeker, and Elijah at Shota's mercy. Unknowingly, both vampires find themselves heading for the Black Hills where Zedd and Shota both believe a portal back to Mystic Falls exists. That is if they can survive "crazy ninja chicks", guards in chainmail (so last century), and dacra attacks.