I've played two games of Descent. One in which the three player game had two heroes, Avacyn and the Thief --
Avacyn's Surge Ability is he can add a stun condition to an enemy, which we read -- and did incorrectly. Stun doesn't remove your whole turn, though wihtout thinking it through it sounds like it does. Stun works as read: The only action you can take while you have a stun card is discarding the stun card. If you have an action after the discard action, take it. If the Overlord plays 'Dash' on his turn to give the monster who's stunned an additional action, basically shakes off the stun for free. If you are afflicted with both Poison, Stun, and Immobilized -- the first action you must take, is shake off the stun by discarding the card.
This is why 'while you have this card' is on there, I think. To denote that no matter what action you take on your turn, removing the stun card MUST be the first thing you do. For example, Avacyn gets stunned AND immobilized on his turn, as well as lethally damaged for 9 hearts. On his turn, he's going to want to use his Heroic Feat, but first he must remove the Stun. Then he can either try to muscle out of his immobilization OR use his Heroic Feat.
It's spelled out as far as the rules are concerned if you look at the purpose of conditions, and the meta-ruling here. It seems to want the stunned condition to be resolved before any other condition is resolved or any other action is taken on the turn. Either Hero OR Overlord.
I completely agree. My group was confused about this at first, but I thought about it later and it hit me. They HAVE to word it like this to cover the different ways you can receive Stun, and how it can affect you in those particular situations.
For instance, you are stunned while it isn't your turn. Your turn starts, you read the card which says: Discard this card or token. However, notice it starts with an 'Action Icon.' Even this is a clue. If it was to simply take your whole turn, there is no need at all for this symbol. it clearly indicates it takes an action to do whatever you are about to read. In this case, it takes an action to discard the card. After this, it says "This is the only action you may perform on your turn while you have this card or token." This is telling us two things, and the reasons are obvious. The first part is there because it is one of the only ways to indicate that you must perform this action before any others whilst not knowing exactly when you receive this card/token (this is where covering the different times you could get the condition comes into play.) The second phrase is there to tell us that once you've satisfied this limitation, it no longer applies to you. Because it makes you use an action, and indicates it must be the first action (which is to remove the card itself) it becomes clear that you only lose 1 action. The reason they had to make it read like this is again because of the different times in which you can receive the condition. The other clear example; Pit Trap. If a hero is using fatigue to move around and the OL plays Pit Trap on him (assuming he fails) he is then Stunned because he has no movement points to suffer. The wording of Stun is necessary in this situation to reflect the possible moments this could happen during a hero turn. Say the Stun card simply read "Lose your first action and discard this card." This is no longer simple when the hero has received Stun because he used Fatigue to move, after taking his first action to Search, over to attack a monster. However, consider the way Stun IS written and you'll realize that no matter when he suffered Fatigue to move (before, in between, or after his two actions), he'll lose his next action and ONLY that action.
This got kinda wordy… and for that, I apologize.