Eeps. I'm not too sure whether this screws things around a bit; modifying the original theory slightly:
I had one of Erio's (the poster) Extended Relic Weapons recover a Relic that held a scale of Void Dragon (the name Nel goes by with the Old Belka) and Jail uses the main gun of the Cradle (which creates rather than destroy) to power the ritual to summon Nel (my pet theory about what Jail really was up to).
The ERW and the Relic were the seeds, and everyone manages to interrupt the summoning, but what remains is a portion of Nel's full form which had been activated.
To boost what had been unsealed, Nel draws Keroko into a gem (she's worn herself out by then to resist) and uses the magic she had accumulated as his own; Makes the piece of himself much stronger, and everyone think twice before nuking the Avatar of Neltharion with everything and the kitchen sink. I know it's cheap.
And to be sure, that ERW is VERY evil, having been Project F'd from a now-dead Aspect Mage who conspired with the Void Dragon. >=3
I believe I still need some more changes, need your input for now.
Well, I suppose it's not impossible, but instead since he's evil, it might be better to make him a normal willing Aspect Mage rather then another shard. That would keep Keroko's uniqueness, while giving the enemy the needed powers and an extra level of evilness too (after all, if you manage to get in good grace with Neltharion these days, you need to be pretty damn evil).
Erm... How against are you to another shard of Neltharion appearing in fanon? He's quite powerful, needing all CCs and most OCs in cooperation to take down; I'm trying to stick as true to his Warcraft roots as possible, but the way I work him might be going and destroying your work. Any hints to help...?