Magma Dragons never had an interest in mortal conflicts, the petty humans and their affairs were insignificant, their mortal lives shorter than a sigh and mostly spent on foolish conflicts.
Murglun, even at her young age, knew this; she lived with her kin in the bowels of the earth, where the lava rivers kept her warm and the conflicts and power struggles of the surface were meaningless.
From the moment Vez'nan knew these creatures existed, he knew he needed them to tilt the fate of the battle to his side. He forged a plan to control them, he knew these creatures were mystically connected to the volcano heart gems, and searched far and wide until he got a hold of one.
Now Murglun has to obey his command and use her fire powers to scorch the heart of his foes until this battle is won and she can return to her lair.