That isn’t true. Thralls cannot use the Sword of Crom or any weapon that is corrupted, which includes Carnage and Blackheart Blade, but every other two-handed sword with 80+ damage can be used by thralls.
This is no longer true. Post Age of War, Sword of Crom merely has a 3x stamina drain multiplier. If you manage to get your stamina high enough, you can actually swing it multiple times.
Thralls were prevented from using Sword of Crom because it was only supposed to be swung once before you depleted your stamina, but this restriction carried over even when it is only a stamina drain multiplier now.
This is inconsistent though, as other weapons also have stamina drain multipliers and have no such restriction, which is why I suggested thralls get a “Stamina Rating” to balance these weapons out without giving thralls actual stamina.