Ok im not sure about the exact details of this spell so any help about how it works would be appreciated. But please only statements that you're absolutely sure about It's quite important to me that the information im getting is correct
the following is the assumptions about imp invisibility im having, correct me if im wrong
Until you attack/cast spell/and so on, you are invisible and cannot be attacked and have bonuses to attack someone since you're invisible
When you attack you're still considered invisible in most aspects and will keep your bonuses to attack but will now be attackable and actually "visible" but you would have 50% miss chance to all attacks made against you
"When you attack you will become visible to all enemies, no matter if they're actually within visual range. As soon as you attack/cast/whatever you will stay attackable to all until you cast a new imp invisibility
If you search the threads on this forum you will find a number of them that address an issue/bug that emerged regarding the Imp. Inv. spell in the game.
The spell is supposed to work as you mentioned (or close enough). Unfortunately, the bug has turned it into what is effectively an invulnerability spell. After you attack, the enemy still cannot "see" you to hit you. So you can stand there and whack away or cast spells until the Imp. Inv. expires without any danger of reprisal. So, the spell does not work as written.
**Spoiler**Spoiler
This "can" have a huge effect on the final fight in the game. As one of the villians has and uses this spell. See the walkthrough if you want some help with that.
I re-read some other threads and I saw that K-monster mentioned that this bug was fixed in a patch. I don't know where to find it but shouldn't be too hard.
It's quite important to me that the information im getting is correct 