Using the Invisible spell will turn you invisible and many creatures will not see you. This is a useful spell.
Some creatures are also able to become invisible and some creatures are able to see invisibility.
Note that invisible creatures do not appear at the battle window and only area attacks affect them. The best choice is usually GFB.
To make a creature visible again, you need to cast the Cancel Invisibility spell or cause damage to them (for example, creatures that are immune to Fire will not be made visible by GFB runes).