Mutating Warpblade.

[Belial69]

Just re-reading the daemon codex and came upon a large slice of cheese that I'm guessing will get FAQed.
Warp mutation: If the bearerslays an enemy Charecter or Monstrous Creature, roll a d6: on the roll of a 2+ the victim is transformed into a chaos spawn...etc.

Now, the other weapons have a tendancy to specify a situation,
Bloodlust: bearer has rampage.
Decapitating Blow: Any to wound rolls of 6 made with this weapon...
Warpdoom: If a... Last wound in close combat.
Contagion: ...suffers an unsaved wound from this weapon.
Touch of rust: Weapons with this ...glance on a 6 etc

You get the idea, so if a model has the mutating warpblade, as it on specifies that if the bearer slays, not when or how, would that mean that if my herald of tzeentch blasts someone with magic, or say a LoC with hellfire gaze etc, would a spawn still be created?

RAW says yes.
RAI, well its tzeentch so would make sense I guess.

[Tynskel]

You are supposed to slay the opponent with the weapon.

[RGilbert26]

Not sure how you are coming to this conclusion. It's quite obvious how the weapon works and its effects only work in close combat after killing a character or MC.

[Belial69]

It comes from some older gw rulebooks where there was a difference between bearer and wielder. I am under no illusion that it should say when the bearer slays a character or mc in combat. I was bored at work and found yet another lack of gw proff reading and figured a bit of rules lawyering would pass the time.

[RGilbert26]

Meh. I Don't see any lack of proof reading.

[Tynskel]

Yeah, I think you are trying too hard.

[Belial69]

Yeah, re-read it after a good nights sleep, lol. It was a very slow day at work, and after 9 hours you can pick holes in anything. Although there is a slight oversight in the proof reading. I copied that word for word, they spelt character wrong, lol. Like I said slow day.