The attack cards say you can move from "anywhere on the board except Valhalla".Huh, I would've gone with GSReis' response, but it turns out, this IS allowed...
If a province was full with say two of my guys and two of my opponent's guys, could I play an attack card that targeted that region, and move one f my guys in that province from his current village to one of my opponent's village? Or, does he have to come from another province or even another region?
Page 6 of rulebook (emphasis mine)
These cards are special because they allow you to move into a
village already occupied by an opponent’s warrior. When you do,
the enemy warrior is displaced and sent to Valhalla.
Take one of your Vikings (leader or warrior) from anywhere on
the board except Valhalla and move it to a village in the kingdom
indicated on the card, or to Asgard or Vanaheim. If you move
to a village that is occupied by a warrior, move that warrior to
Valhalla. You cannot attack a village occupied by a leader (leaders
are tougher than warriors). If you move to a heaven, you may
move one warrior (not a leader) from that heaven to Valhalla.
It is usually best to attack using the Vikings from your boat, but
it isn’t necessary. You can even attack using a Viking already in
the target province or heaven if you wish