Can this Power be stacked twice? I had a game that I held two such cards in my hand. If so, does this mean I can spawn the free monsters TWICE whenever a new room opens up?

Yes, if they reveal a new room you can place up to 4 extra monsters.

There's no rule that you can't stack them. The two Cards have to be in play to actually take effect, and not only in your hand. Keep in mind, that you can only play 1 Power Card per turn.

One last question on this matter....

If I have Horde of the Things already in play and the party reveals a new area, can I use another Power (also already in play) called Brilliant Commander to upgrade one of the spawned monsters? Or am I only allowed to upgrade an existing DUNGEON monster and not a spawned one? This is because the dungeon monsters usually suck, and one of the spawned monsters could be a beastman and using the upgrade, I could get an easy command aura into the room of monsters.

I really don't know the answer to that second question. The wording on the cards is entirely unhelpful, as whether you can or not depends upon which order you resolve the cards in. I don't remember any FAQ or similar on the subject, though someone else may be able to correct me.

From a rules as written point of view, I think I'd be tempted to answer "no", as in the absence of any other evidence, it would seem reasonable to resolve the powers simultaneously if possible, and if you do this, the beastmen aren't in the area to be upgraded at the time Brilliant Commander is looking for a target. But if you think the overlord ought to get to choose the order (something of a precedent for that, as heroes are usually allowed to choose the order of things affecting them), then it looks like the answer would be "yes".

But really, so long as you get agreement among the players, it's not going to matter much which way you play it.

Maybe this helps?

Q: Can the Overlord place the second monster granted by Hordes of the Things adjacent only to the first monster, which is in turn adjacent to a revealed monster? Is the Overlord allowed to place the granted monsters outside of the newly-revealed area if one of the newly-revealed monsters is adjacent to a previously-revealed area?
A: Each monster is placed individually next to a revealed monster. Each monster must be placed next to a monster which was on the board before the Overlord used the Hordses of Things effect on the newly revealed area.

Maybe not? You can't use Hordes spawn to help you place Hordes spawns, but this is referring to spawns within a single card. Maybe the above doesn't count if you have two Hordes cards in play? I would tend to think that your second Hordes spawns can't be placed next to your first ones, but that's not directly addressed by the above. I would also think that, in part due to the above, your Brilliant Commander can't be used to upgrade Hordes spawns. Flip a coin, though, I guess. I play you can't use Brilliant on Hordes spawns.

I would actually go the other way. In general the consensus has been that when two things happen at the same time the owning player gets to choose the order that they reslove in. To me that would indicate the choice is up to the Overlord. Check the FAQ for confirmation if you want as well. As far as the question of using monsters spawned by a previous horde of things as a base for the next horde, that is a totally different question. My gut reaction would be as they are on the board before resolving the second horde's card that they could be used, but I do not have any rules basis for this interpretation.

Brian

Based on all your feedbacks, I guess I can conclude that there is no right or wrong here.

In that case, I would legalize or disallow the upgrade based on the OL's win-loss ratio. So since in my case, the adventurers are still learning the game and they have been losing 6 quests in a row, I will not allow my games to upgrade the spawned monsters using Brilliant Commander. Thanks everyone for helping to brainstorm.