1) Californian metal band
2) Slovak metal band
3) Polish dance band --- 1)
Diadem is a Californian metal-band which was founded by Anthony Prechtel. The label (his label) is Studio December, which was renamed to Studio November.
However it is difficult to classify the band further, since Diadem is influenced by various genres, so you might identify elements of Viking, Black and Death Metal, but they merge quite impressively.