Here's a reason to emerge from your post-holiday stupor and maybe even put on pants: Trolls! No, not the angry waitress at your local diner—actual trolls. Scandinavian gnomes, too. Are you intrigued yet? Gorge upon fish, oats, grains, oysters, game, lamb, ham, turkey, cheese, overwintered fruits and roots at a Scandinavian "Once in Valhalla" 10-course feast at Somerville's Center for Arts at the Armory on January 25. It does sound rather epic, and helmets are involved.