From Path of the Vampire Wiki
Lyngvi/Bifrost can be found via block Station squares under Bifrost, or north road in world area going past Cherridale.
Canonically, these enemies are essentially spirits of ancient fallen warriors who believe have been given powers by a mythical being, and have been tasked with claiming the souls of dead warriors, and returning them to their master whom they believe to be the god Odin.
Hildr = 113 -125 xp
Loot: Mostly Valkyrie Dust (value = $50) rare drop; Hildr Battleaxe long-blade
Acc: 11 Pow: 15 Qua: 8 (Value = $3000)
Valkyrie Sword long-blade
Acc: 15 Pow: 9 Qua: 7 (Value = $2500)