After reading more about the upcoming patch it got me thinking again.
Mannaroth's skull is hanging literally right above his head. Thrall used to have it in a secluded area of Orgrimmar, away from himself or anything else important. Did he know?
Demonic skulls; Demonic artifacts even have been known to influence their owners.
Illidan for example was possibly influenced by the skull of gul'dan, but gul'dan was just an orc warlock. Mannaroth was a pitlord. THE pitlord.
What could be more devious, more clever than for his spirit to live on in the skull, and bring ruin to the son of the very orc who killed him?
It's so beautifully perfect. What better way for Mannaroth to exact revenge as well as weaken the horde for the next legion attack? Since we're probably looking at a legion expansion after this, it just fits so well.
Garrosh's pride over the legacy of his father will be his downfall. I love it.