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But in the case of Railag, who first broke away from the rule of one, all-powerful Overlord (six princes are six princes; in an internecine war, it is often not the one who was initially one of the leaders who wins), then won a similar internecine war, taking advantage of one of the weakest dark elf Clans, and then outsmarted (though not without the help of other heroes) the leader of the strongest of the existing alliances on his own turf.
1) The knowledge he gained during his service to the Overlord, and especially as his right-hand man, gave him a significant advantage over the other dark elf rulers.
2) He almost died when he refused to serve Kha-Beleth. He was saved only by a miraculously found heart.
3) He heavily relied on the support of Tieru, and one of the strongest demonesses helped him.