Could be someone within the involved racial factions (goblins (Ashstalkers, ???,....), hobgoblins, orcs, trolls (mercenaries but I doubt those two companies were the only ones), drow and likely others). The bloodstone could mean Yves is involved for some reason, they seem like they should be very rare and valued objects, beyond the reach of tribal goblins (plus they would definitely have used it before this if they had it all along. They are not that diciplined). We know Yves has three from dealing with a trio of sanguimancers in the past. The king may have given one to the goblins in order for them to quickly take out the kobolds and as payment for their role in the war. Lots of other options.