We have official word on the Mythic Chrome king Gallywix kill! It seems it was indeed an exploit, as Blizzard explain what happened.
It seems the new accounts got hold of “an internal spell”, which sounds like a GM “finger of death” type tool, and killed some raid bosses. If they had more GM tools at their disposal, getting to Gallywix wouldn’t have been hard. Blizzard stopped the cheat and banned the accounts.
We immediately began an investigation into these unusual kills, and we detected a group of new accounts using an exploit to cast an internal spell to kill Mythic raid bosses, including Chrome King Gallywix.
Our security engineers quickly moved to put a stop to the cheat and take action against the accounts involved, and we have cleared the Hall of Fame to await the rightful winner of the RWF.
Thank you to everyone who reported this. We will continue to employ the utmost vigilance about fair play in World of Warcraft.