Seen in different worlds
but never quite like this- Angra Mainyu is in Final Fantasy XI as the chief of the area Dynamis - Beaucedine, also the Ahriman is seen in other Final Fantasys as a winged demon with one eye.

Hidden by the forgotten remains of the past,- You find the fiend in the...damn I forgot the name of the desert but I'm sure you know the desert I'm talking about. The "forgotten remains of the past" refers to all the junk machina you find in the desert and probably under the desert as well.

you may find me within evil sight- This refers to the relationship between Ahriman and Angra Mainyu; in Zurvanite Zoroastrianism Angra Mainyu is some type of destruction spirit god type thing. I didn't read all of the wiki page on the religious beliefs and the history of angra mainyu but I discovered that it was also known as "Ahriman" Zoroastrianism and it is also the middle persian equivalent of Ahriman.

I am the destruction of the past,
I am the spirit of the dead- These two lines were to show that it was a fiend; the destruction of the past meaning someone dies and after a while the spirit of the dead or the pyreflies combine to make the fiend. Also Angra Mainyu was refered to as a destructive spirit in the Avestan-language.

If you want to take a look for yourself
Angra Mainyu - Wikipedia, the free encyclopedia heres the link

Not to shabby eh? My best riddle yet So it's your go BG