There are four different bonus games within the main game itself. Each of these has different awards and is triggered different ways.
The Valkyrie Bonus Game is completely free and is unlocked for everyone. It awards the player 10 free spins and multiplies all wins by 5 during the free spins, which you can re-trigger. If 3 or more Bonus Hammers emerge, an extra set of free spins will be awarded, on top of those you already have.
The Loki Bonus Game can be played once the Great Hall of Spins Bonus game has been activated 5 times and awards 15 free spins. When one of the famous Wild symbols appears on the third reel, any number of regular symbols, to a maximum of 14, are converted into wilds. Free spins can be re-triggered.
The Odin Bonus Game can be unlocked one the Great Hall of Spins Bonus game has been activated 10 times, at which point it will grant an extra 20 free spins. Any win while in this bonus game will trigger the Wild Raven Bonus feature. Here, symbols are randomly transformed into wild multipliers. Players can be awarded up to a x6 multiplier on the amount you win, so long as it contains a wild card. However, this feature cannot be re-triggered during free spins.
The Thor Bonus Game is only playable after the Great Hall of Spins Bonus game has been triggered 15 times and rewards the player with 25 Free Spins. Any win while in this mode will trigger the Rolling Reels feature, where the symbols will explode and disappear after the player has been paid out. The symbols that are above the exploding icons at the time will fall to accommodate the now empty spaces and award the payout with a x2 multiplier. The multiplier may then increase in value by one to a x5 maximum. This game cannot be reactivated during Free Spins.