Is it possible..

To do The Ebony Citadel Of Mallyx and then get the reward and start it again?
so is it repeatable?

Thanks in advance <~~

Yes, it is repeatable, but once you have claimed the reward you must start from the beginning again, i.e., clearing all the four prerequisite areas, then revisit Mallyx. You cannot just hit the last part over and over again (and a glitch that allowed some people to do that was fixed and the perpetrators permabanned).