Collect 5 Collector's Books, from the Collector's Book Quest, or dropped by Obsidian Assassins, and give them to the Centaur King, located in the Centaur City.
Rewards:

• Lion's Ring
• 3,750 XP per level

In total, 1 Lion's Ring will have cost you 375 Moon Tug Scalps!

You can complete this quest at will.