Agni Invades is the first of the Arcana Release Quests you will get. You will first get a notification that you are sensing Arcana power, and the Veddels will let you know that you can now start the first Arcana quest.
The first time facing him, you can only attack his body. You can't lock on to his right arm just yet as you need the Eye of the Gods Orb to be able to identify breakable and weak parts. It' still possible, though difficult, to break his arm on the first try.
Defeating Agni unlocks Ahbor Desert and Chapter 2 quests.