In another thread people are talking about a Ring of Improved Invisibility. How then does a Ring of Invisibility work? When you hit someone you become visible? I don't understand...
A ring of invisibility works just like the DMG says. You activate the ring and it gives an effect like the spell Invisibility. So yes, after you activate the ring, you'll turn visible if you attack someone.
Nothing. However, activating it requires a standard action. Thus you would only be invisible every other round. And when you're visible, the other side can attack you back.