Crescent is from the Alcyoneus galaxy where the moon Eos of the planet Elpis once was. The Ori had attempted to take it over, but when the people of Eos refused, their leader had destroyed both Elpis and Eos for not conforming to their ways. By burning her people to the ground until there was nothing left. She was caught in one of the fires as a young child, and she was badly burned to the point that she should've been dead. But, someone had shipped her to the Pegasus Galaxy to Atlantis, and the Ancients had ran experiments on her to see what they could do to save her. Her body was so severely injured, however, that there was no way of saving it. There wasn't anything left from her body to attempt to save, the damage had been done.

Until, one suggested that they make her a synthetic shell. One that her brain and heart are able to power it. They used very complex technology from their own, along with Asgardian technology to make this project possible. While they did this, they had kept her body in a containment unit to wait until her mind had grown enough, since it could've been dangerous to do so beforehand. She was in a comatose state from both shock and the machine from when she was six years old while they worked on completing her shell.

That is until the Wrath came to Atlantis and destroyed them. Her mind and heart had been placed into this body, but her brain wasn't synchronised with the shell, so she wouldn't be able to move when woken up. From then, she was trapped inside of the pod for a very long time until she was woken by explorers from Earth that found her and they worked on being able to finish what the Ancients started with the help from the ones at SGC. Since Dr. Jackson knew more than they did.

They did this by creating an antidote that's taken daily to synchronise her brain with her shell with the help of the Asgard, being that their kind took part in creating her. They however either don't know, or never said what happened that this was the only option for her survival.