—Description in Fire Emblem Echoes: Shadows of Valentia.
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After Celica's agreement with the dark priest Jedah in Duma Tower, Alm and his forces are freed from Dragon Mountain and push onwards to Rigel Castle. There, the Deliverance lays siege to the castle, hoping that their victory will bring an end to the war with Rigel. They encounter Emperor Rudolf during this battle, but he refuses to fight Alm, resulting in him being struck down. With his last breaths, Rudolf reveals Alm's true identity as his son and heir, Prince Albein, and that it is Alm's destiny to wield the Falchion within Duma Tower and slay Duma. Following this announcement, the people of Rigel Castle, including General Massena and the remnants of Rudolf's army, hail Alm as the true emperor.
Not long after the death of Rudolf, Alm encounters Mycen in the castle, who reveals the passage in the basement that leads directly to Duma Tower. Alm enters the passages in an attempt to find and aid Celica, and fulfill the destiny Rudolf revealed to him.
Meanwhile, in shock and anger at the news of Alm's identity, Prince Berkut is drawn from the castle into the basement passages by Duma, who entices him with promises of power as a way to right the wrongs that had been done to him. Rinea and Fernand both separately enter the passages in search of Berkut. However, all three meet their end in the passages thanks to Duma's sway over the prince's mind.