Medeus was once a prince of the Earth Dragon royal family, but when the dragons were forced to take Manakete form, he was the only Earth Dragon who complied and thus did not go wild. After the other Earth Dragons were sealed by Naga, Medeus was tasked with guarding the seal.
Over time, he became furious over how humans looked down upon his race, so he formed the Doluna Empire and enslaved humanity. He was eventually defeated by the hero Anri, and his kingdom was destroyed. Centuries later, the dark priest Gharnef brought his forces back and worked to resurrect him. Medeus was revived, but not completely, as he could not leave Doluna Keep. He was eventually slain by Anri's descendant Marth.
Later, Gharnef tried to revive him again using the noble bishops: Lena, Elice, Nyna, and Maria. This time, Medeus metamorphized into a Dark Dragon. However, Marth killed him before he could do anything further.
“Foolish humans! You dare disturb me while I slumber? Oh, how you will regret it as you die slowly...painfully...writhing in the fires of the inferno!”
—Medeus' battle quote in Shadow Dragon
“Hmm? So... I am not the only dragon revived. I had heard you perished the last time. No matter; you've recovered even less of your power than me. This time, you have no chance!”
—Medeus' battle quote versus Nagi
“I... HAVE RESURRECTED... STUPID HUMANS... I WILL KILL YOU ALL...”
—Medeus' battle quote in book 2 of Mystery of the Emblem