The icy skeletons were summoned by the Bringer when he was awakened by the Thieves Guild on Terror Mountain. Like most mindless monster hordes in the world, they went around aiming for general destruction, their ranks being one of the sides in the Battledome component of the Hannah and the Ice Caves plot. Those that appeared were:
The Icy Skeleton Hordes also feature in a random event that steals a few of your Neopoints.