I don't know enough about DF modding to know if this is possible, but...
What if Voidspawn were somehow butcherable?
What if one of their products was a void-heart made of a modded metal?
What if we could collect void-hearts and forge voidsabers and voidplate for our military?
Would be badass.
I didn't do much testing with Overlord-level voidspawn, most of my testing was soldiers. Voidspawn soldiers definitely beat some major ass.
I also found that they're *highly* arrow-resistant for the most part. I tested a 5-voidspawn soldier squad (naked/unskilled) against competent arrow militia armed with iron crossbows and plenty of bolts, and it took like three ever-increasingly-large waves of crossbowdwarves to finally kill the voidspawn. I think we can definitely injure and slow them down with crossbow legions, but they're so damned resilient that I'm not thinking bow spam will save us unless we set up, say, an underground megabunker with every available civilian packing iron/steel bolts.
And we've seen what so much as a Drone can do once released among the beards, considering that a single snatcher was enough to score quite a few kills.
I think the scariest part, though, is that damned taint. It kicks in fast enough that anyone who just manages to stalemate a voidspawn is actually more of a threat to our military than someone who dies instantly on contact, since the voidspawn seem guaranteed to win any extended engagement just via the abyssal taint/eventual enthralling of the opponent.
Do thralls automatically spawn wearing the original dwarf's gear? Do they get the dwarf's skills? If A or B are true, then danger rooming recruits might be the exact worst thing we could do...