If you know what the different tunings are, it's a Drop D tuning, but then an extra half step down.
If you don't know, the tuning is called a Drop Db(drop because it's going lower and a D flat). You tune your biggest string(what's usually the E above the A) to a C sharp, then the string below that into a G sharp, then the string below that into a C sharp, and so on and so forth.