Røkkr Sieges is a game mode for Fire Emblem Heroes.
In this mode players face off against giant shadowy versions of Heroes called Røkkr. These shadow Heroes have a huge HP pool and players must team up with others to defeat the Røkkr.
Players expend mode exclusive Stamina and send select Heroes into battle to defeat the Røkkr within seven turns or by dealing a certain total of damage to the Røkkr. Heroes may directly attack the Røkkr, dealing massive damage. Occasionally, enemy reinforcements appear on the map and the player can defeat these units to gain a damage multiplier. Directly attacking the Røkkr also grants a small multiplier.
The script for this mode can be found here.
Røkkr is Old Norse for twilight