Spell Casting in Melee

In Melee a spell is cast on the magi's QIK, because they don't have an Init Modifier. Is that Init + a stress die, or just QIK?

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A maga's Initiative is based on the Quickness alone (p. 174, third column at the top). So it would be Quickness + Stress Die. And, unlike 4th edition, a magus's spell goes off on their turn in the initiative instead of at the end of the round. So, if they're in a combat, their shield grog has an Initiative of 3, his foe has an Initiative of 12, and you have an Initiative of 5, the sequence would be Foe => Magus => Shield Grog.

And magi with spells capable of responding to attacks can attempt to intercept those attacks with a fast cast defensive spell. They can respond if they achieve an ease factor of the initiative point of the attack on a Qik + Finesse + Stress Die roll. Their ability to respond is limited to a spontaneous spell or a spell specifically mastered for fast casting. The Casting Total is reduced by 10, as well, so spontaneous spells have a a serious drawback... But sometimes life and death situations can be avoided by a judiciously cast spontaneous spell.

That all seems to have been covered, but as I suspect I know why you're asking...

Under RAW, the magus is likely to need a concentration roll, given the clamour of battle around him. You could stick to RAW and have a concentration roll for each spell, opt for rolling only until he hits a success (at which point he has enough composure to carry on), or leave them out totally.

And last point from me, remember that characters can declare that they are defending other characters, in which case the defended character can't be directly attacked - it has to go through the defender first (don't have my books with me right now, so you might want to check on any limitations to that).