If it looks ornate enough, i'd let it be a relic blade.
Personally, i converted mine using the Marauder/Beastmen kits, the head is the mace/scythe thing, whilst there's the mace from the flail on the bottom, the staff is actually the banner, complete with hand (he was originally a librarian, and i prefer to think that he needs to be able to physically touch the weapon to use it as a force weapon).
If it looks good enough, then use it.
Also, the description for a frostblade states it is in essence a chainsword, yet i've never used a chainsword for one, far too hard to convert, especially when a power sword looks far better in most cases.