Well, it doesn't HAVE to be Ballos, although that seems like the obvious choice...
Want to know who else it could be? Take a second... think...
... . . .. . . . .
Who do we know that talks using Full Screen Talk...
Who do we know that had his body destroyed, yet lived even after death...
Yes! The Doctor!
Are we certain that the Doctor is dead? We broke his body and he was able to possess the Core to battle us.
But with the Core's destruction he simply disappeared in an explosion... Was that enough to kill him...? Or...
Maybe with the Core's destruction, Ballos' magic was free; meaning maybe the Doctor was drawn into Ballos' hurricane of magical energy? Then when Ballos was finally destroyed, all that was left was the remaining wild magic... including the Doctor...
"Help me"
"It hurts"
"So hot"
Maybe now the Doctor is being crushed by the terrible power Ballos was forced to endure for so long since Ballos is gone.
If this is true, I don't know what this might mean... if the Doctor will go insane and continue to exist by possessing things? Or if he will just burn out without Ballos' body producing more magic energy...