Slayers are skilled with all manner of axes and carry a mixture of single- and double-handed axes around with them. At the start of any combat, a Slayer unit may choose to fight either with two axes counting as having an additional hand weapon, or to take a single axe in both hands, counting it as a great weapon. If the Slayer has a rune axe then this rule ceases to apply and he fights with the rune axe only, generally as a hand weapon but possibly as a great weapon if it has the Master Rune of Kragg the Grim inscribed on it.
Once they have made the choice on how to use their axes they must continue doing this until the combat is over.