I was fighting an ancient dragon and then it happened, it gave me its soul during combat while affected by dragonrend but it still was killable afterwards. does it have something to do to the fact that it was affected by dragonrend. or is just possible it will become unkillable.
Dragonrend just forces them to land you shouldn't get their soul before they die, the soul should leave the body as their flesh burns away. There are a few reports of that happening around though so it must be a very rare bug, some people have said it couldn't be killed after so you may want to reload if you can and maybe see if it dies normally.
I had that happen the other day. In the middle of fighting a dragon, it burned and gave me a soul when the dragon was at around 75% health. I couldn't hit it until it flew around a bit and landed again, at which point I was able to kill it.