"I felt that I should warn you about what could happen once I accept the blessing of the Goddesses," Flayn told the two of them.
"Warn us?" Dedue asked.
"There are two possible things that could happen," Flayn said. "One is that I will fall asleep and you won't be able to wake me. Don't worry if that happens, that means the blessing worked. I will be unconscious for weeks, but Nayru promised I will awaken before the attack on Hyrule Castle."
"For weeks?" Dedue asked. "Will you not need to eat and drink?"
"Manaketes can sleep for years in order to heal from major injuries," Flayn explained. "When I was initially injured, I was unconscious for almost a millennium. And no, I won't need to eat or drink during that time. Though I will likely be hungry when I awaken. Don't worry, it's not dangerous for me."
"You mentioned two possibilities," Revali said. "What's the other?"
"Well...the other possibility is that I lose control of the power being granted to me, assume on overcharged version of my draconic form immediately, attack Calamity Ganon, and fight until either Ganon kills me or the power does," Flayn admitted. "If that happens, there won't be anything you can do to save me, so don't try."
"And you still plan to do this?" Dedue asked, horrified.
"I have to," Flayn said. "My draconic form's powers are a perfect counter to Calamity Ganon's. We need the edge."
"But if you do this, you could die!" Dedue protested.
"And if I don't, we could all die!" Flayn shouted. "There's nobody else who can do this in my place. I have to go through with it."
Dedue looked like he wanted to protest further, but he bit back his objections and stood up straight. "Very well. If this is what you must do, then so be it."
"Thank you for understanding," Flayn said, and turned to return to the Spring of Wisdom.
As she walked to the Spring, Dedue followed behind her. When she looked quizzically at him, he said, "Unless you do not need to breathe while asleep, you may need someone to fish you out of the spring before you drown."
"...Good point," Flayn said.
The two of them walked into the Spring of Wisdom, and Flayn looked at the statue of Nayru. "I'm ready."
Edelgard and Lysithea had described their blessings from the Goddesses as the single most physically painful thing they had ever experienced. That wasn't the case for Flayn. Oh, it was painful, to be sure. But more than anything else, it was the single greatest rush of energy that she had ever experienced.
Instinctually, she wanted to put that energy to use. It took a lot of concentration to keep in mind that that was exactly what she should not do. She struggled to control it, as it struggled to overwhelm her. Her grasp on it was tenuous, and she didn't know if she could hold it all in.
But before she lost control, she felt Dedue's hand grab hers.
With that, a new burst of resolve filled her as she thought of the people she couldn't stand to leave behind. Her friends in the Rising Sun Company. Father, Rhea, and the rest of her family. People throughout both Hyrule and Fodlan whom she had helped, or who had shown kindness to her. People who didn't, or wouldn't, want her to die today.
With that burst of resolve, she felt her entire existence cling to the energy that threatened to burn out of control, and force it fully into her being.
Victory!
"I did it," she thought, as she stumbled into Dedue, rapidly losing consciousness. She felt him catch her and pick her up right before she fully fell into a deep slumber.
Maybe this chapter shouldn't have been separated from the last one, I'm not sure.