Icetusk Trolls are a new one introduced in Bfa with no stated origin.

Actually, there's a chance this cloak drops from the island expedition. You can take the cloak to a Tuskarr in Northrend and he'll tell you the history of the tribe. Apparently they lived in Northrend until the Ice Trolls started to lose ground to the Scourge. Instead of fighting, they decided to flee and sailed south. This didn't work out well for them as they were unable to find a home and many died. In their desperation, they then turned to the very necromancy they had fled from in order to raise the dead members of their tribes so they could still fight to secure a new home for the survivors.