Jagar Tharn is the final boss of The Elder Scrolls: Arena, a powerful Imperial Battlemage who usurped the Emperor. He is a master of all schools of magic, capable of casting devastating offensive spells, powerful defensive wards, and debilitating curses. The battle takes place in the Imperial Palace after you've reassembled the Staff of Chaos. His primary weakness is sustained physical damage, as his magical defenses can be formidable. Focus on closing the distance and engaging him in melee if you are a warrior. Mages should use their most powerful damage spells, prioritizing those that bypass magical resistance if possible. Be prepared for a long and arduous fight, constantly healing and dispelling his effects. The Staff of Chaos itself, once fully assembled, will be your greatest weapon against him.
