The Rygnar Idol is an event that can be found in Chamber of the Lost Idol in Diablo III, and can appear as a bounty. The Chamber appears in The Stinging Winds, and always contains the event.
Upon using the entrance portal to the Chamber, heroes will be asked by the amateur treasure hunter Poltahr to help him find the legendary Idol inside the ruins. The Idol was once in possession of Zoltun Kulle, and finding it would lift Poltahr and his family out of poverty. He will then become a companion for the duration of the quest, as long as within the Chamber.
Inside the ruins, which are filled with Undead minions, one can find Poltahr's Notes, and ultimately the Idol. Once touched, it will summon four Skeletal Guardians (of either type), and once those are dead, may be taken, ending the event, rewarding gold and experience.
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Diablo III - The Rygnar Idol Event Guide