I had this little quirk come up in a game earlier where I played ACB and my Greyjoy opponent simultaneously played Valar. If I choose my plot to resolve first and target his Iron Mines, what happens? Does he get to use it anyways, or is it nullified before Valar goes off? I believe that he would still get to use it since using its ability would not change its moribund destination, but I'm not sure.

tibs3688 said:

He can't use it. The problem is not the moribund destination. What the rules forbid is removing a moribund card from play a second time. The Iron Mines requires the triggering player to kneel and discard the card as the cost of the save effect. If The Iron Mines has already been discarded by ACB (it still being physically on the table in a moribund state notwithstanding), it can't be discarded again as part of the cost of its effect.

If it was The Iron Cliffs, your opponent could trigger it from moribund, of course, because there's no discard cost.