Ragnar-O's is a dungeon & raids achievement earned for defeating Ragnaros while three Living Meteors are ignited by Lava Wave.
Trivia[]

Two scoops, Executus! Two scoops!
This achievement's name is a reference to Wowhead user TheCheat54's comment on the [20-30D] A Binding Contract page on Sep. 6, 2011, stating "I want to eat Sulfuras. It would go well with a fresh bowl of Ragnar-O's." This in turn refers to one of the contract's terms, stating that the Thorium Brotherhood is not responsible if the signee was harmed by attempting to eat
[Sulfuras, Hand of Ragnaros].
The comment inspired an image of a Ragnaros-themed breakfast cereal that began making its rounds on the internet the very next day.
Patch changes[]
Patch 4.2.0 (2011-06-28): Added.