You can get one by beating the final boss on fjordur to do that you have beat beyla the bee boss steinbjorn the bear boss and hati and skoll, killing them gives you relics which you use to fight the next bosses the megapithicus, the brood mother, and the dragon killing those bosses gives you there heads you use the heads to fight the fenrir boss when you kill the fenrir boss you'll get mjolnir a skin for the tek sword that you need to be lvl 190 to use and a fenrir in a cryo pod if you beat fenrir on alpha you get a better fenrir then if you beat gamma,

The fenrir takes inventory acts as a refrigerator and if you do (LT on xbox) it does a bite attack which deals a decent bit of damage, and if you press (right joystick on xbox) it does a ice explosion that does area damage and makes spikes come out its body and gives it added defence, you are able to breed fenrir if you have a mating pair, and they get a pack boost if you have multiple.