The blade of realities is fantastic. However, you need to realize that your defenses will be poorer because of it. At best you will just have the 1+ armor save if you are mounted, 2+ if you are on foot. No wards, no rerolls. That said, T5 1+ AS is still pretty good.
If you are going to run it, I would be sure to put it in a unit. Either a cold one bus to give a LoS, or a decent sized unit of anything else to at least stop ballistic shooting. Magic support is nice too. High magic works wonders with Apotheosis and Hand of Glory.
When you do get into combat, it is a total monster. Roll to hit, roll to wound, and you are done. No parries, no armor, no wards. S5 is pretty good for wounding most things. I took out a High Elf noble on a star dragon with this guy, no problem.
So, it's good, but you are giving up some of your defense in order to have that power. It really messes with Tzeench players, though. Take that, "unkillable BSB"!