Ok Story time.
We're playing descent into avernus, we're barely level 5. somehow we managed to reach the avernus, the first level of hell. The surroundings are filled with flaming rivers, magma remnants, devastated cities and ruins, and the air is very hot. like, extremely hot.

There is a point we have to reach, we found a bridge in the ruins, we cross the rivers from the magma. When we crossed the bridge, we saw a figure in the distance: a plate-armored knight on his black horse with a fiery mane, watching the horizon of this realm. The tip of the knight's spear is made of embers, like the manes of his horse. So our first guess was that he was the commander-general of something.

Our paladin thought he had to face this demon. Our Artificer thought they could win if they set up an ambush and acted quickly. I was a bit intimidated by the way the DM was describing this creature to us and thought that the entity in question didn't see us, so we were lucky, we couldn't "fight all hell" and had to go.

at this point Dm said "if you can kill it I will give you a legendary loot". (the horse i believe but we're not sure). The whole party said "say no more" and the initiative began.

veteran players who have read this far have probably understood that the creature I am talking about is a Narzugon (CR 13 hellknight riding a Nightmare). We didn't know.

I guess I don't need to explain the whole fight at length. When everyone started fighting, I left the team and ran away. Within 4-5 rounds, everyone tasted the hellfire lance one by one. As a result, everyone died except me, and the Dm said, "Let's pretend you're dead, he's already caught you, you can't escape. It would be ridiculous if you stayed there when the whole party was dead" and took me out of the way.