Rule set. What in dear name keeps you invisible or unable to see. I mean you walk in front of some one and they can't see you for some reason.
And rule sets are not lore friend, an ability or a move is not powered by you rolling a 20.
Added, Ultima's world has no way of making or powering these items, why are they even there.
Seriously, with video game rpg in combat realistic and making sense are never ground lines. to what is put in combat.
I just explained that too you. The game shows you as "shaded" when you are in that mode. The more people in the room, the higher the DC of succeeding, etc.
Just saying that, in most rpgs, what the lore says is possible, is thereby possible in combat.
Read the lore or play the games, and you will find out why they are there. They disapeared after the third game too, I belive. With good reason.