I don't think you can trigger ragnorak spawning by joining others. Unless someone can verfiy. I've gotten Ragnorak once and it was from my own raid. Also get 50 jewels for initiating it once so you want to at least once.

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Also, very weird. I'm not sure if I broke the game but joined a raid, game crashed, logged back on and couldn't find it in progress/pending. Went ahead and joined another raid without problem. Finished it and when I went back to the homepage, it said I had a raid in progress, the one from before. Lol, not sure what happened there.