[psycho835]

Were they ever stated? And if not, could anyone give me their approximate Force?

[MijRai]

They've been statted previously, yes.  Page 32/33 of Hazard Pay, they are Force 15 Wraiths, with minion-type weaker spirits and the Astral Gateway power to acquire more in a tight spot.  Standing bounty on destroying (not disrupting) one of them is 350,000 nuyen; good luck with that!

[psycho835]

Thanks, chummer. Wish me good hunting! :-P

[psycho835]

Say, how do you even destroy a Shadow Spirit? Couldn't find anything in "Street Magic" nor "Street Grimoire", beyond the fact that they are all Free Spirits. Is there a "Shadow" metaplane to banish them to? If so, can they come back?

[8-bit]

They use the same rules as Free Spirits. They can be banished like Free Spirits and destroyed like them. They return to whatever their original metaplane was, whether it's a "Shadow" plane or just another metaplane, who knows. The main thing is, if you banish them, they can come back like normal Free Spirits when their Edge replenishes, and they will likely be really angry with you. The only way to destroy them is the same way as destroying a Free Spirit; find their True Name, get a copy of the Spirit Formula, and then attempt to destroy it. Here's the quote with the rules for a Free Spirit.

A free spirit that has been banished or disrupted can be prevented from ever returning if the banisher immediately destroys a copy of the spirit’s formula and succeeds in an Opposed Test between her Banishing + Willpower [Mental] and the spirit’s Force + Edge. Unfortunately, there’s no way to tell whether this succeeded or not, unless the spirit later returns. The banishing attempt may be repeated, but this requires another copy of the spirit’s formula.

Just be wary of obtaining a Spirit's Formula, as they know where the copies are, and will likely search out and destroy any owners of a copy.

A spirit is instinctively aware of the location of all copies of its spirit formula—where any of them are when they are created, moved, or destroyed—no matter how many astral barriers exist between the spirit and the formula. If a new copy is made, the spirit automatically becomes aware of its location.

[MijRai]

Say, how do you even destroy a Shadow Spirit? Couldn't find anything in "Street Magic" nor "Street Grimoire", beyond the fact that they are all Free Spirits. Is there a "Shadow" metaplane to banish them to? If so, can they come back?

You can banish them if you try hard enough (Force 15 with Banishing Resistance is no joke), or get rid of their local form by blowing them up with magic and missiles.  Then there's disrupting a spirit, which works just like any other spirit; you find its home plane (Yeah, good luck surviving that trip), track down the spirit formula, and use that to destroy it.  Otherwise, after...  I think 28-Force days, they're back. 

[Marcus]

Beating a force 15 in a straight fight is doable, no joke, expect causalities and fatalities. Completing a force 15 astral quest is may not be impossible, but it's not something I'd ever contemplate trying To really manage it you would need a whole team of mages, and giant pile of magic support.

[psycho835]

So, treat it like a bog standard Free Spirit. Got it.

[The Wyrm Ouroboros]

... ooo, tempting.  Force 15, you say?  And a metaplanar quest to kick its ass on its home plane?

... heh.  Hm.  Heh heh heh.

[MijRai]

... ooo, tempting.  Force 15, you say?  And a metaplanar quest to kick its ass on its home plane?

... heh.  Hm.  Heh heh heh.

Enjoy!