I kinda think the fight with Shujinko killed him.
But no, he was never given a backstory or spoken about. He's just supposed to be the REAL Champion of the Elder Gods, to hint that Shujinko is being tricked and his quest isn't what he thinks it is...and after you beat him, the next real Champion of the Elder Gods ends up being Scorpion.

Would be interesting if they brought him back as an actual character.

All I know is Monster was originally going to be an alternate costume for Scorpion in Deception, it was his supposed knight outfit. Before the game's release Boon and company felt that it didn't fit Scorpion and so that idea was scrapped.

I'm still not completely convinced to this day that he wasn't (and still isn't) just supposed to be Scorpion, just as Ghost was supposed to be Liu Kang... but the sloppy writing and messy timeline presented in Konquest mode made it unclear, So he kind of took on a life of his own. But I personally never thought he was intended to be a new character, like some.

However, it's been brought up before, and there are people on this board who will fight the notion tooth and nail, so now I'm somewhat scared to have even mentioned it.

But regardless, he was never important enough to give a second thought. He's a vague encounter with Scorpion at best, and a generic pseudo-nameless NPC fight at worst.