I am sorry for not specifying(though I am suprised to see so many replies).
I have TotSC + latest patch + DSotSC + BG Harder Patch. By the information gathered by what you guys said, it is most likely that this used to be a robe for neutral characters or such but modified by one of the TeamBG mods I have. I retrieved it from a mage that was with 2 others - The first people to meet me inside the Ice Dungeon.
I'm pretty sure this isn't your ordinary Robe of the Evil Archmagi. It was a significantly enhanced version of it. It's effects included +5 AC(not AC 5), Saving Throws +5, Increased number of spells +2 to lvls 1-5, +1 to lvls 6-9(the numbers might have been slightly different, but I'm sure it went up to lvl 9) and -30% Constitution.
This item sounds ridicolously powerful, but then again, that's why I want it
Thanks.