This is from the novel
In the year 1, Draka gave birth to Go’el, child of Durotan, shortly before joining her chieftain on his secret meeting in Loch Modan with their old comrade Orgrim Doomhammer. There Durotan explained what he knew - about Gul’dan, the Shadow Council, the demonic bargain and Orgrim decided it was time to stop the evil warlock.He urged Durotan and Draka to take shelter in the north until he sent word to them. Orgrim commanded his guards to accompany the Frostwolves on their journey home and keep them safe. That would be a mistake that Orgrim would regret to his dying day. His guards were not loyal to him, but to the Shadow Council.
The guards had heard every word between Orgrim and his visitors. They decided they didn’t need Gul’dan’s permission to eliminate the Frostwolves - clearly, he would want Durotan, Draka, and their child dead. After traveling north for several days, the guards fell upon Durotan and Draka. The Frostwolves fought bitterly against their attackers, killing one of them. But the remaining guards struck down Durotan and Draka, and Go’el was left to die in the frigid cold.
They were clearly guards loyal to Gul’dan/The Shadow Council. Not sure where you got your info about them being assassins.