Actually, Louisoix's class at that point was literally Primal (specifically, Phoenix). Thousands of Eorzeans had channeled aether from the land into 12 stones, each bearing one of the symbols of the Twelve, and from there, Louisoix channeled that energy to the Carteau Flats. He told everyone to pray for a reborn Eorzea, one that was no longer facing the Garlean threat. When he failed to use that energy to contain Bahamut (the mini-Dalamud with the giant symbols of the Twelve surrounding it, which Bahamut broke apart like paper), he lost control of that energy. Since everyone on the battlefield had their attentions, hopes, and dreams focused tightly on their commander, Louisoix, they all unknowingly channeled all of that aether that was created by wishes for a realm reborn into Louisoix, causing him to completely transform into a Primal relating to rebirth. Then he tears Bahamut apart as seen in the cutscene, and it's Bahamut's death (and the crashing of what's left of Dalamud) that devastates the land and causes the Calamity, not Bahamut himself. And then Louisoix tries to release the aether that is now his body back into the land, but.......