The 'enslavement' route would be my preferred way to go. The purity thing is deep into her origin story, a major issue the CS only touches on. Essentially this would work like ripping the very darkness/evil out of a Grimm (applicable to evil people as well except their aura greatly protects them from the ability) If a part of the darkness is taken the Grimm is weakened. If a significant amount of darkness is taken a Grimm may be unable to harm a Hunter at all, but could still defend or flee. If every bit is taken the once evil creature is now pure. it will obey Skye and even willingly act to the benefit of humans. However, a purified Grimm would begin to die, gradually. Grimm that were more powerful or excessively evil would even deteriorate quite rapidly. So Skye could not purify and build an army for humanity out of Grimm. Also the limitations on how many she could purify, how quickly, and for how long would make this only useful for skirmishes, and ideally against weaker Grimm.

More importantly this power [i][u]scares the ever-living [b]SHIT[/b][/u][/i] out of Skye. Besides background reasons think about it: where does that darkness drawn out during purification go? But now I'm kind of exited to take things in this direction.

[quote=@Crimson Raven]

I think that has some interesting potential.[b] A powerful semblance that only targets Grimm? That would go a long way to helping explain why Beacon would take in such a ragged and troubled girl to train.
[/b]
Perhaps she could even temporarily take over and manipulate Grimm, able to make them fight for her. And then destroy them when she releases them. 

Or she could be inheritly dangerous to them, her semblance could weaken Grimm that stay around her or cause them to become fearful and flee.

Lots of possibilities here.

Personally, I rather like the first one.
[/quote]

Too true.