To get to Wintertusk, you need to finish the main quests for Grizzleheim, until you unlock Ravenscar at level 40. After you beat Rsvenscar, you unlock Wintertusk, Just a warning, there is no level requirement for Wintertusk, it is very difficult and is meant for wizards level 60 and up, I unlocked it at level 45, and I needed help all the time. It is worth it, you get a lot of xp.
It is worth it, you get a lot of xp.
Hope that helped!