This really is a challenge in need of a name so if you could suggest one it be great. Anyway the idea is...

After seeing his youngest son off to his first year at Hogwarts Harry Potter collapses. He is rushed to St. Mungos and after an exhaustive battery of tests the results are not good. Harry has cancer, while magic might be able to cure many things an advanced brain tumor that's been slowly growing for the last sixteen years is not one of them. Harry is given three months to live. He takes it rather badly, after all the average wizard lives to 140 and the healers say he won't make it to forty.

So here the challenge begins. Rules.

This is a post epilogue DH story so everything should be in cannon.
-No matter what Harry will die in three months, no amount of magic can save his body.
-But then again Harry knows quite a bit about how to cheat death, or more specifically survive the death of your body.

This can be done in a lot of ways but the main thing I was thinking about is Harry hating life and by extension the world for what's happening to him and willing to delve deeper into the dark arts to stay with his family.

 

You're missing the motivation. Harry was already ready to sacrifice himself once. It's against his character to be afraid of the death to such extent that he turns into something he's been fighting his entire life.

 

It's testicular cancer.

 

The motivation is he wants to remain with his family. I don't see it as a plunge into the deep in but a gradual decline as everyone else gives up on finding a cure and when his attempts to use the Deathly Hollows fails he remembers Horrocruxs. Remember at the end of DH he really didn't have anything to lose as he hadn't truly lived yet.

 

You might want to rephrase that. Remember, in canon Voldemort said that he'd never felt death, even though he'd been torn from his body and the only reason he survived was because of his horcruxes.

I just can't see any ... real thought in this challenge. So Harry becomes sick and emos around trying to find a way to stay with Gin-gin and the his family who he loves so much? I just can't see this as a DLP kind of story; too much angst and too little explosions. Even so, someone might decide they like the premise of it and take it up. I'm just one opinion.

 

Okay, there's a bit of a problem with your challenge.

You want a post-DH, epilogue-consistent story about Harry gradually turning dark in a desperate attempt to stave off death and remain united with his family . . . . fine, I guess.

Your Harry has three months to live after the Epilogue. Again, fine, I guess. It's a better fate than to remain married for upwards of fifty more years to a Weasley.

The problem arises when you combine these two elements. A completely canon Harry, even with a complete Bellatrix-esque psychotic break into full-on Dark Lord Syndrome is not going to accomplish anything in three months other than rack up ungodly amounts of badly-written angst, a la Annakin Skywalker in Revenge of the Sith. I doubt that even if he immediately conceived the idea of creating a horcrux, he would be able to muster up the chutzpah to even attempt it in three months, much less actually succeed.

 

I can see Vash taking this one. Since he has nothing to lose, Harry decides to spend the rest of his life having fun, and... er... leave a few small pieces of himself behind, a la Final Half-Year.

 

Immortality via genetic dominance of the wizarding world.

 

So he makes lots of babies?

I like it!