Sort for the late reply. The skull was infested with the soul of the yellow one and by taking and wearing it the grey one became like a Capri sun to it. After draining his life force the soul had grown as big as he once was but the grey one died. These souls only come from traumatic deaths and will be forced to stay attached to their bones unless the bones are destroyed

Oh no no, this is too sad...
What exactly happened after the grey one took the skull of the yellow one? I don't understand...