From Hulk Vs.
The Hulk shall be sent back to Midgard, and then you shall answer to Odin!
Fine! Take the creature! It provided me with amusement enough, seeing him pummel you nigh-unto death!
[Amora discovers Bruce Banner has been slain by Loki
You... you fool, you've killed him! Do you know what you've done? Because the mortal died here, Hela claimed his soul! Without his soul, I cannot return him to Midgard! Without Bruce Banner... the Hulk cannot leave Asgard!