Shepard didn't think she would ever open her eyes again after stepping into that light. A light so bright that she could feel its heat from steps away. A heat which provided comfort she allowed herself to admit internally that she needed. She was so tired. Tired of fighting endless fights, of trying to convince people too stubborn and proud to come together even when the horrors were destroying everything in front of them. Horrors she spent years warning them about.
"They will stand in the fire and complain that it is hot." A quote from one great character in an old game she found time to play when she had just joined the Alliance. That quote had given her the necessary push whenever she had to deal with an unthinkable situation or stubborn people; and Shepard had dealt with many of both. She vowed to never be that kind of person, that she would always do what had to be done, no matter how hot the situation. She would never just watch and complain, never be idle in suffering.
And she wanted it to be over. Once and for all. For every life lost, in this and past cycles. For the future lives never having to suffer through what they had. Never having to worry if some horror would come and hunt them to extinction.
With thoughts of a better future she stepped into the green light, hoping that the reapers had been honest and that it would provide true understanding between the parts. And that it would end. Enough war, enough suffering. They deserved to live. She had lost so much, sacrificed so much. If a last sacrifice could save them all, she would do it.
And despite what the Crucible told her, using the face of the child who had haunted her throughout the months, she once again did the impossible.
She didn't know how. Maybe having interacted directly into the Geth consensus had unconsciously taught her how. Maybe it was her superior biotic control over matter. Whatever the reason was it only mattered that she survived.
Enveloped and consumed inside the green beam, Shepard was deconstructed down to atomic levels; her conscience, her body, everything that made her what she was, was absorbed, analyzed and distributed through the massive relay network and from each of them transmitted to every reaper, organic and synthetic forms, uniting them all.
And after everything she still had the will to live.
The Commander recombined herself; and it was a new body. Nothing in the galaxy was like before; everyone was a hybrid of organic and synthetic, and so was she.
Shepard didn't think she would ever open her eyes again, but she did. And she had friends to find.