Fighting style largely borrowed from Siegfried's fighting style. It utilizes somewhat slow, but high damaging attacks along with stances.
Mostly same as in Soulcalibur, with a few changes.
Nightmare's fighting style has changed since Soulcalibur II, though still similar to Siegfried's fighting style, it has become faster and more brutal. Between the two characters, Nightmare would be more of a "Berserker", with fast and powerful attacks. The drawbacks of Nightmare's fighting style would be that it leaves openings in or after a number of his attacks, leaving him open of a counter if the opponent is fast enough.
Same fighting style as in Soulcalibur III.
Nightmare's Soulcalibur III move set was improved upon in Soulcalibur IV, notable changes include:
He is the only character in Soulcalibur IV with a form of Soul Charge.
His moves also seem to flow much better than in III.
Quite a few moves have different inputs and animations.
He has some better close range punches and kicks.
Nightmare plays the same as he did in Soulcalibur IV, though he does have one new guard break attack which can be performed by inputting the R button (default control settings).