Odin as the Dark Rider is mentioned in the quest from president Naja to increase mercenary rank. In Wajaom Woodlands you can find "Dark Rider's Footprint" or something like that on the ground. That's about all I know. Nobody I know has fought him or even knows how he's spawned. The other HNM that I know of so far is Cerberus who's found on Mt. Zhayolm. A large hnmls on Shiva fought him for ~7 hours but didn't have enough ppl to actually kill.