Enrage Level I needs to be purchased before Level II, and so on.
Players are limited in the level they can purchase by the level of their Mage class - you must have Mage 4 to purchase Enrage 4, for example.
Enrage increases your base (raw) Attack values (the bold white numerical Attack value shown at your Keep) by the indicated bonus percentage. This also affects your Effective Defence values at the normal 70% rate.
Enrage can be removed (reset to 0%) during battle, even with a defeat, by the Rogue ability Intimidate