Source: LiSA's Crossing Field Album
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Itsuka no Tegami (いつかの手紙, Itsuka no Tegami, lit. "The Letter I Once Wrote") is the second song...
The Sword Art Online anime is an anime adaptation of the original Sword Art Online light novel...
Aincrad Arc Opening Theme: Crossing Field by LiSA, Closing Theme: Yume Sekai by Tomatsu Haruka...