—Description in Fire Emblem Echoes: Shadows of Valentia.
Dead Man's Mire is a massive swamp in Rigel. It is extremely poisonous and most citizens are advised not to wade through the bog unless absolutely necessary.
Celica and her party travel into the swamp on their way to Duma Tower when they are attacked by Terrors. Jedah contacts Celica telepathically, informing her that both divine dragons are degenerating and he can save both if she sacrifices her soul to Duma. Having no reason to believe him, Celica declines sacrificing herself and her party continues onward.