Assuming you had 5 packs of darkhounds that can only be killed by baelfire, What would happen if you unleashed them on a large army of say... 100,000 men?

Could they somehow be contained or would they eventually wipe out the army?

If they would eventually wipe out the army, why have we not seen them used like this before?

And if they're so BAD A$S then why has the shadow used them more up until now?