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After Hiyori calls Yato from Yomi, he used Yukine to cut the "Pacification Ring" and they played a song that is so nice.
If you're talking about the flashback scene that starts around 4:59, this is "Reincarnation" (Noragami Aragoto OST, disc 1, track 8). Here is an extract from the track (0:45-1:00).
It's the 11th song on the soundtrack Footsteps. The portion we hear during that scene starts at minute 1:17 and crescendos around 2:00 when Yato uses Sekki.
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