@drache64: One? I feel One is probably the most he is going to be able to take, and imo, its more going to be from the surprise factor of the Space Marine not realizing how potent Ghal Maraz is and not understanding just how potent dwarf forged gromril is and so Sigmar being able to pulp the Space Marine when he smacks into his armor and finds out the chainsword doesn't do anything. Any more then one and it becomes too much as they are just too fast.
@decaf_wizard:Depends on the individual. It eventually gave Azhag human intelligence and the ability to cast Death magic. But he probably wore the crown for sometime, Sigmar didn't wear it all that long but when he did, he was stronger and a better fighter then when he didn't have it... at the cost of slowly driving him insane.