The Barents Observer has been doing their best to figure out what's going on, starting from their first report:

July 2 
Fire onboard nuclear-powered submarine, 14 sailors killed

July 3 
Fishermen witnessed nuclear submarine drama

July 3
Defense minster confirms fire onboard the «Losharik»

July 4
Report to the President: super-secret submarine «Losharik» will be repaired and taken back in service

The fire started in the battery compartment, but did not affect the reactor, Defense Minister Sergey Shoigu reports to Vladimir Putin...

Tragic, and potentially disastrous for the immediate area if the reactor casing had opened but not something you'd call "a planetary catastrophe".

Among other reports we've seen (not verified so grain of salt) is that seven of the dead were captains, meaning whatever they were up to was pretty important.