Invisibility
Invisibility is, in most cases, a simple spell that in later Eras will be classified as Illusion. Of course, that spell can be detected by anyone who knows what they are doing. Whether or not your character will be detected will be a matter of agreement between you and the players around you, depending on your characters' power levels and magical prowess. Please be reasonable about it!
You can display invisibility by using the Kollopi Essence, which can be obtained by any Dominion character even at level 3.
