NOW LIVE! -- One-Page Adventures for D&D 5th Edition on Kickstarter! A booklet of colourful one-page adventures for D&D 5th Edition ranging from levels 1-9 and designed for a single session of play.
That's pretty much up to your DM. You could always say your character did not know about his heritage, and that up until now his life had been pretty normal. Now with adventuring and the stress of trying to stay alive these other qualities are starting to display themselves.
At its most basic level, a template is just a bunch of mods that are applied to a base creature. The backstory about being the offspring of a fiend is there for flavour, and isn't strictly necessary to use the template itself -- you could easily substitute something that just involves spending an extended time in the presence of fiends. For example, a PrC involving demon summoning could have as its top-level feature the character changing into a half-fiend.
Now, I suspect what Magic Rub really wants is some sort of _published_ method of turning into a half-fiend. I'm not aware of any such, though.
"The blackguard empitomizes evil. He is nothing short of a mortal fiend..... " (DMG P. 30) I think there would be a higher chance of achieving this goal. But just MO.
He is probably thinking of bastion of broken souls, I allow it. You have to kill a celestial and drink the blood of a fiend and use a wish to complete the transformation. For celestials it is kill a fiend and drink the blood of a celestial, use a wish. For a dragon you have to kill the opposing dragon type and drink the blood of the type you want, and use a wish, etc.
In one of the recent issues of Dragon there was a series of Death Knights from Greyhawk. The Leader of these is a Paladin turned Blackguard who worships Demogorgon. IIRC he became a Half-fiend as a gift from Demogorgon and this was accomplished through the Prince's "demonic surgeons." A pretty gruesome way to gain a template, but if you want it real bad...
It could be that the forums aren't churning out new threads each day, so scrolling through the list a little bit brought OP to one they're interested in replying to.