There are several bands named Mysterium. 1-Mysterium (Romania):
Timisoara based Latin language Black Metal project MYSTERIUM was assembled in 1996, issuing the demos 'Templum Tenebrarum' in 1997 and 'Odes Pro Malis Ingeris' during 1999. With Kujcsen joining on keyboards, the band switched title to EKASIA for the EP 'Umbra Si Reflexe'. 2-Mysterium (Germany) :
Mysterium a dark/black metal group from Germany has produced two albums under Prophecy Records.

Read more about Mysterium on Last.fm.