Magus appears first as a boss in Chrono Trigger. You set out to fight him thinking he’s trying to revive Lavos. But in reality it was something different.
He is an ancient being, whom was a prince of a royal family who just happened to summon Lavos.
So as Prince Janus he was sent into the future with his life ruined. He tried to summon his nemesis to extract revenge upon him.
And he actually joined your party as a very powerful warrior. As your end goals were the same.
I thought it was ironic that his mother, Queen Zeal was still alive and as a vengeful and powerful demigod.