Jan Erik Tiwaz, aka Tyr (born 22 June 1972) is the former bassist for the Norwegian black/folk metal band Borknagar. Tyr is known for his progressive playing style in black metal and his use of erb's (extended range basses). Tyr made of use erb's in his later work with Borknagar, namely a 6 string fretless on 'Origin' and a custom 8 string fretless on 'Universal'. Tyr has also been a touring bassist for Satyricon, as featured in their live video Roadkill Extravaganza (2001) and with Emperor as seen in Emperial Live Ceremony (2000). He currently resides in Drammen, Norway.
During June 2010, an announcement was released that due to "general disagreements", Jan Erik Tiwaz left Borknagar.