Cube all 3 parts in Hell Act 5 to create a Red Portal to Uber Tristram where you'll fight 3 Uber Bosses at once for the Hellfire Charm and Standard of Heroes.
Be prepared. This is extremely difficult.
Some gameplay tips:
Baal's Eye is dropped by Uber Duriel in Forgotten Sands as part of the Pandemonium Event. Uber Duriel is perhaps the easiest of all uber bosses.