The official site for the Pokemon Movies (here, in Japanese) has updated and revealed some small tidbits about the plot on the ‘Director’s Comments’ page.
The movie takes place inside of an extremely cold mountain which has an abandoned steelworks inside. The sacred swordsmen, Cobalion, Terrakion, and Virizion, encountered Kyurem here. They challenged it and Kyurem created the ‘Ice Dragon Gate’, a special battle field.
The director went on to talk about the themes of the movie, saying they were simply focusing on Friendship and Comradery.
<3 pokejungle
ps- zzzz time to sleep so I can wake up at 6am for work v_v