Well, what do you think it is?

3277903 How about an alternate Dark Equestria where everyone can use forms of dark magic. And if he gained all the Light Equestrian magic he would be able to conquer both worlds.

3278151
Not bad!

3277903 Actually I just finished writing a one-shot that touch on that topic, All I'm missing is a cover so that I can upload it.

Let me try:
1) Tirek and Scorpio were once a humans from the MiddleAge/Classical time who were transported to Equestria by Eldritch powers that existed before equestria itself and also responsible for the creation of Discord. They gave them these powers and set them to destroy Equestria itself. The rest is history.

'I used to be a regular human, until one day I was bitten by a radioactive horse, and my life changed forever...'

Forgive me, if it feels like I'm jumping the shark on this one.

But I like to think the MLP world use to have the Greek Pantheon. They have pegasi, Cerberus, and other elements of greek mythology, so why not the Greek Gods. Why do I bring this up? Because I think Tirek is one of Zues demigod children! Oh, and when I mean 'greek mythology'. I mean, real greek mythology. Where Zues is a womanizer with LOTS of demigod children, Hera is his violently jealous wife and sister, Hades is actually a not-so-bad guy, and so on. The world (or the setting) has a 'repeating history' vibe, with the son overthrowing their tyrannical father only to be just as bad. Rinse and Repeat! Guess what Tirek did!

It would establish why he's such a badass, and make his 'Magic should be MINE!' reason, a bit more fleshed out. Also, he's a centaur! Meaning he's supposedly half pony, half human. Though, I doubt that, considering his more primate looking appearance.

But yeah, this is just some flimsy back-story I've entertained here and there. I like to imagine him as a lone traveler, who challenges the most powerful of beings in the lands he happens to travel too. Stealing their magic. Would it be a stretch to have him be the reason there's so few Alicorns left?

4202884 Hmm, that's actually pretty interesting! Celestia and Luna are already kinda Apollo and Artemis... and Cadance is Aphrodite... I like it!

3277903 The IDW comics have recently come out, you might find one or two surprises in its pages.

FIENDship is magic Issue 2 Tirek