A Timelessly Hapless Mage (Dropped)

All the time, many people learn of legends which a mage becomes immortal. Throughout history, there have been thousands of geniuses who have gained immortality. No one pays attention to those behind them, the stepping blocks, the antagonist, the "evil" one. Protagonists just laugh in their faces, knowing that evil "plot armor" will protect them. What if, one hapless mage, who became the antagonist through various means, succeeded? This is the story of your average neighborhood villain, who became immortal though being a "secondary" character.