I believe there is a necro NPC in The Gathering Storm (which I dont have yet), and I would like introduce him in an earlier adventure. So my question is: what sort of powers and stats does he have etc.? what's his story? Is it even plausible to ham him appear earlier?

His name is Lazarus Mourn. His backstory is fairly strongly rooted in Stromdorf and he's actually been caught and executed already and is coming back from the dead in TGS.

Unless you're intending to use him in a MUCH earlier adventure, then I don't think he's suitable for what you want. However, there is an in-built recurring NPC mechanism in the adventure which means he can keep on coming back in different forms. (As a side question - did anybody's group clock what needed to be done with the pendant, or did your group just leave it too?)

Having said that, Lazarus Mourns stats, actions and spells make for a perfectly respectable generic Necromancer if you're getting TGS.

When you have an "artefact" that needs destroying and can't figure out how, you hand it over to Sigmar's temple

First, your numbnuts PC's have to realise that there's something odd about said artefact...which they don't (despite your fantastic and clue-ridden descriptions).

thanks for the answer. hmmm.... maybe I should just get TGS an jump on into it. It just sound very combat heavy, which I dont mind, but I dont want the campaign turn into a hack'n'slash experience.

I'm going to have my players run into Lazarus pre-TGS before his death. I'm changing the story a bit in the process.

My Lazarus Mourn turned to Necromancy in a mad attempt to learn how to resurrect his dead wife. His Twin children have been captured by a particularly nasty Witch Hunter who isn't squeamish about using them to lure out the vile Necromancer.

I plan on having the PCs meet the Witch Hunter and making him and his treament of the children really unbearable. The children will be as scared and as innocent as I can muster. Then I'll have Lazarus find the PCs and try to convince the PC's to free his children. I expect my players to be split on whether or not to kill him on the spot or try to convince him to turn himself in if they can convince the WH to free his children.

I'm also toying around with the idea that the Innkeeper killed Lazarus' wife also. That could partially explain why his wife was running around on him in the first place.

That being said, I won't be able to run TGS exactly as it is written.

Am I evil?

There are a number of ways to introduce Mourn without having the characters actually meet him.

In my game, I'm referencing him early by having a cultist using his writings. One of the cultists from Eye for an Eye escaped to Aurswald, to meet up with a former fellow cultist. That contact long ago left the Unblinking Eye to pursue his own studies in Dark Magic after finding a cache of Mourn's notes. He now seeks the "key to immortality" that will eventually be revealed in tGS (not having Mourn's skill, the ex-cultist has worked up to a similar, unrefined method).

I'm also tying in much of the adventure from SoF by having the experimentation causing the outbreak of Ghoul Pox.