@shroudofsorrow: Meetra simply defeated them. In the case of Sion, he repeatedly dies and gets beaten by Dun Moch, as per cutscenes. For Nihilus, it lists him as "being no match of them" and "falling in battle", which is more than enough to indicate it was by virtue of convenient battle.
For Traya, it says that she was more powerful but was beaten regardless, which is obviously an instance of combat skill.
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