What's your rating on this backstory?

[NecroDancer]

My character is a Teifling warlock named Sympathy, his parents worshiped Graz'zt and as a reward Graz'zt promised to bless their child with his abyssal powers. Growing up Sympathy's first name was Akmenos, life was horrible but then when he was 15 a fire broke out in the city and he escaped in confusion. Akmenos was badly wounded and when he woke up he was being tended to by a kindly cleric and his apprentice. After hearing Akmeno's story the apprentice dubbed him "Sympathy" in jest but Sympathy decided to keep the name. Throughout the next 10 years Graz'zt gave Sympathy more power in hopes of corrupting him, the cleric realized this and taught Sympathy to use his powers to kill demons and Devils (Graz'zt was not pleased). When Sympathy was 25 Graz'zt finally killed the cleric and his apprentice and took their souls. Graz'zt then offered to bring them back to life if Sympathy pledged his loyalty and killed an innocent in the name of Graz'zt. Sympathy refused knowing that the cleric and apprentice's souls would still belong to Graz'zt either way. Sympathy than swore to kill Graz'zt. Graz'zt than used his abyssal connection with Sympathy to shorten Sympathy's life span to 1 year. Know Sympathy travels the land trying to secure power to kill Graz'zt, also Sympathy indulges in almost about everything to try to "experience" life before his time runs out. Sympathy's current plan is to make as many pacts as possible in order and then kill his patrons or cause a supernatural battle from all the claims to his soul when he dies in hopes this also kills Graz'zt.

So Demons and Devils beware, Sympathy stalks the land and he will kill you and your cult after taking your power for his own.

(Personality: crazy and power obsessed, greedy, and probably a nymphomaniac, but willing to stand on the side of good and kill fiends with a smile on his face)

[Falcon X]

A few notes before my opinion:
- Graz'zt probably wouldn't care if Sympathy killed a bunch of demons and devils. Definitely not devils. He MIGHT care if they are higher tier demons under his service.
- Graz'zt might take precautions to make sure you didn't ever get a chance to harm him. He could do a lot to you pretty easily, seeing as your pact probably involves him having access to your blood and possibly even your truename. Of course, he probably isn't even worried that you can harm him. If he limits your lifespan, it will be out of pure spite until the point that you actually enter his domain.
- Fiends are really, really smart. They've lived a very long time and observed humans for a long time. It might be possible to trick your way into another pact or two, but the majority will be able to tell that you have a contract with someone else. You will certainly get no more than one Devil pact. Demons are more chaotic and might not spend the time checking if you've had one before. I'd say you might get one or two more there.
Odds are, if you get multiple pacts, they already have plans to exploit it. That's where life gets REALLY interesting.
- How to get more pacts: Do not try talking to the main demon. Talk to a younger upstart several spaces down the hierarchy who might be eager enough to get your soul that they would make mistakes.
Start talking to Yugoloths (Daemons, Neutral Evil Fiends). They just like pitting the Devils and Demons against each other and might help you get your way.
- Love the personality. It'll be fun to play.
- Even if you didn't get many pacts, the idea of having one year to party it up and slaughter is a pretty great idea.

8/10 rating, which is about as high as I give. This character will be a blast to play. Just think through some of the feasibilities I listed above, and once you are confident that the backstory checks out, I think it will be a lot of fun.