User blog comment:FairyRaven/Fairy Tail Chapter 400: Wings of Hope, Review and Commentary/@comment-9595671-20140913110701/@comment-7418318-20140913114246

@FTGM: And with his failing body and no female around to reproduce, he might have had no choice but to take over Rogue's body, but Rogue's body wasn't strong/old/resilient enough, so he had to wait. And when Rogue's body was ready, his mind proofed too strong to simply take over, so he's slowly erroding it from within.

And the reason why he could fully take Rogue over completely in the fight against Gajeel was because the present Skiadrum and the one F-Rogue brought with him from the future worked together.

Certainly an interesting theory and plausible enough to maybe even be true.

@SDT (I hope you don't mind me abbrevating your name. If say tell me and I'll stop): Maybe the Dragon Lacrima are something like the Eldunari from Eragon. There a Dragon had an organ called the Eldunar or "Heart of hearts". If they detached it from their bodies, they could transfer their mind and soul into it upon death and it also retained some of their powers... though in that series they couldn't actively use it without a living being as a medium.... more or less.