The Karakal Council and the Mantis ancient heroes occupied a rather special position—or rather, all Mantid did. After all, their race was a derived variant of the Aqir. Had their ancestors not migrated en masse back then, they would still be living deep underground in Silithus.
After breaking free from the Old Gods' direct rule, the Mantid, like the Nerubians of Northrend, built their own unique civilization. Their average intelligence also surpassed that of ordinary Aqir, those distant relatives of theirs.
So rather than saying they were loyal to the Horde, it was more accurate to say they were loyal to Garrosh as an individual.
The Mantid race had inherited every trait of insectkind. They were tough and cruel. The destruction of the amber tree in the Heart of Fear would not make them lose their fighting spirit, meekly surrender, and cling tightly to some new powerful patron.
The only reason they had joined the Horde, and willingly obeyed Garrosh, was the Heart of Y'Shaarj.
The Mantid had worshipped the Old Gods for generations. After migrating to Pandaria, their object of worship shifted from C'Thun to Y'Shaarj. They firmly believed the Old Gods were their creators, gods worthy of faith.
However, the Mantid's faith was rational. They were not mindless fanatics like Twilight's Hammer. Asking them to follow the Old Gods in playing out a Twilight Judgment, an end-of-the-world scenario where everyone died together, was absolutely out of the question.
In any case, returning to the point: Garrosh, the Horde Warchief, had now obtained the Heart of Y'Shaarj. Day and night, he was drawing upon its power and searching for a way to fully merge the heart into his body.
The Mantid firmly believed this would allow Y'Shaarj to be reborn in another form. Garrosh would be the vessel, just