You have to attack with your action (if able to) and an attack spell require to cast a spell (which you can't do) before the attack take place.
An attack spell has to go through the motions of casting the spell before it goes through the motions of making the attack, and this is usually described as being because the attack is the result of the spell. However, according to the rulebook Attack Spells are described as being "both a spell and an attack" and not as a spell that generates an attack. Therefore, choosing to use an attack spell to satisfy a mandatory requirement to make an attack is legal.
What confuses me though is this statement:
However, the FAQ says that legality is only checked at the moment the creature chooses its action or target. The core rules say that you must make move actions before other types of actions. So you have to be taking a non move action to check the legality of attacking Thorg, and by that point you can't take your move action.
The FAQ does currently have the line "A creature must perform a mandatory action if it is possible to do so when it chooses what action to perform." When I explain creature actions to people, I do tend to break them down into the steps of Activation, Movement, and then Action, however that isn't really the system in use by the game. It is a simplification and it doesn't work that well with this rule on mandatory actions. Instead, the rules state on page 11 of the English rulebook v3, that creature actions work as follows:
1) Activate the Creature
2) Choose what type of action the creature will take
During step 2, you have 3 options:
1) Move and Quick Action
2) Full Action
3) Do Nothing
If you choose option 1, you would immediately check for whether or not you could perform the mandatory action. If you can not, then you are free to do whatever action you wish, and may still use your move before your action as that was a part of your action choice. This is supported by the wording on page 9 of the English Rulebook v3 which describes movement as an action it's own right.
In the end this can be included with our simplified system by saying you check for mandatory actions immediately after activation. However, if you want to go strictly by the rules as written that say you check when choosing your action, you also have to go by the rules as written that say your pre-action movement is part of your action selection and thus would not be forfeited. Both end up amounting to the same thing though.
