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Ren used the White Haven Art.
More specifically, Seraphina's version of it is the one where she summons a construct of white lotus chakra, which then explodes into petals containing the purifying attribute of the art.
She only used this once in front of him, but it doesn't matter; Ren still copied it anyway. It wasn't the first time he saw the White Haven Art in action, but Seraphina's version was indeed better compared to how her martial brothers used it.
While Ren mocked the reputation of the White Haven Clan and exposed their true face, he admits that their main inheritance, the White Haven Art, was good.
This art was created before the White Haven Clan's betrayal of the Banished Arms Clan—during the time when the two clans were still in a strong alliance and the clan heads still recognized each other as brothers.
