My Archrival Brought Me Back to Life

#I open my eyes to find my prematurely dead self lying in a coffin, while the nemesis I've fought my whole life stares down at me with an unreadable expression... What do I do? Help, I'm desperate!#

Blurb:

My nemesis and I have always been destined to be enemies.

We fought all the time as kids. As teenagers, we saw each other as our greatest rivals: if he was there, I couldn't be, and if I was there, he couldn't be. Every fight left us both bloody, and then we'd get dragged off together to listen to the pastor lecture us.

As young adults, the demon race invaded the continent. Thanks to my exceptional talent, I joined the hero's party to defeat the Demon King, becoming the party's mage.

A disaster befell his family, and he defected to the Demon King's army. He accepted the bloodline bestowed on him by the Demon King and became one of his most trusted subordinates.

After that, whenever we met on the battlefield, we fought to the death.

There were plenty of times I almost sent him to the afterlife. He often sent me to the emergency room of a high-ranking priest. We never settled who was stronger, but one thing was certain: only one of us would make it out alive.

Then the Demon King was defeated by the hero.

I died in the final battle between the hero and the Demon King. I cast a forbidden spell at the cost of my life, spending every last bit of vitality to give the hero a chance to kill the Demon King.

At the time, my teammates were keeping him occupied, so he never got to witness my dazzling forbidden spell... Honestly, I was a little disappointed when I died. That guy was a menace, and I died too quickly to drag him down to hell with me.

And so I died.

Then, fifty years after my death, something inexplicable happened.

My nemesis hadn't died. And for some reason, he suddenly... brought me back to life.

Content warning*: Haven't thought of one yet, but there probably is one.

Theme: They're called enemies, but what they really mean is...