The titan known as Rouge Titan screams from it's place upon the Female Titan, the intense burning pain could be felt all the way to his true form in the nape of his neck. Fighting against this he rolls his eyes downwards, to see Annie surrounding herself with crystal.
Somehow he understood what was happening, strange whispers of a memory explained.
The crystal that was now covering her back half and legs was be created by the two titan bodies. Forcing himself to move Eren jerks his head down, the flesh of his titan's body was still fusing all the while.
Eren broke free from his fleshly prison by going straight through the throat of his titan, he dropped down onto the crystal and quickly grabbed hold of Annie's hands linking together with his, he pleases his forehead against hers just as the crystal washed over.
Now the crystal began surrounding him. It all happened in a matter of seconds.
Eren Jaeger and Annie Leonhart was trapped together in the harden crystal.
Later this large crystal was taken to an underground chamber, where it was stood upright. The pair within gave the appearance as if they were floating under water.
While they slept the outside world continued on living. The area around they changed and shifted, the people who watched over them grew old and died. After many years they stopped coming all together, leaving the pair in silent darkness.
The old city above they was tore down many times and rebuild over and over, their chamber was left untouched and hidden as it went deeper underground.
It would be over five thousand years before the crystal began to crack and the two last of the titans awoke from their slumber, finding that everything they ever had known was gone. Or that was how it appeared at first glance.