At the catacombs level 5, go back to town and talk with Griswold. He will tell you about some caravan with strange folks that, years ago, passed through Tristram. They stayed just a few days at the Tavern of the Rising Sun and left. The weird thing is that he heard something about a "magic sky rock" at their carriage.
That caravan headed East, he doesn't know why, and from what he knows, they were attacked and perished. But the things on the carriage are still there. He will ask you to bring him that "magic sky rock" and, if is true that it has magical powers, he says that maybe he can do something useful for the hero.
Backtrack to catacombs level 5 and seek that rock. It will be somewhere there, probably guarded by a horde of demons. Get it and hand it to Griswold for the unique item: Emperor's Ring (very good magical bonuses indeed).
The rock will remain stuck at a pedestal until the Arkaine's Valor Armour quest is completed (if is available during your gameplay).