There is not a lot to think about.
- The humans are known to take goblins as slaves, as Runde's story has shown. If they managed to capture one of our minions, they will hear a tale about the Demon God's return, and then we will have a problem on our hands.
- That limits our diplomacy options, as it means the raiding party must not return back.
- However, we can't use Gigadyne to wipe them out, either, as two Ion Blasts in close succession, with one of them wiping out the scouts, is something that the local lord will consider a threat and will want to investigate with much greater resourses invested.
- The group can't vanish off the face of the earth completely, too, as it would only make the lord/whoever is in charge more curious and they will keep coming.
So the way I see it, we should wipe out most of them, capture the rest for interrogation, seek out someone willing to betray their master or simply brainwash them, and have them tell a tale of how they found nothing in the ruins but were ambushed on the way back by... I dunno, bandits, or a minotaur, or something.
I doubt these guys would be much of an ally, if only because the humanity we know is led by the Goddesses. Also, they have a history with demonkind, and it is not a very positive one:
I assume those heroes were human, given certain tensions between the races.
My guess would be that we are far away from Seven Kingdoms, since we are in a realm that Vaalgrahf was banished from. It isn't surprising that we haven't heard of them, just as Rin has never heard of this place or her ancestral history before the banishment. We are discovering a completely new world here. Well, forgotten would be a better word.
This is also why I doubt they are at odds with Seven Kingdoms, or even heard of them.