亡KEI却KOKU心

亡KEI却KOKU心 (called on the JASRAC database) is Disc 3, Track 5 on the XenobladeX Original Soundtrack. It is split into two parts: the first plays from the start until 2:54, and the second plays from then until the end.

The first part plays in Oblivia during the day. The second part is a more subdued remix of the first and plays in Oblivia at night.

Etymology
The title can be split into the kanji '亡却心' and the romaji 'KEIKOKU'. Combining the first and third kanji into the character '忘' and joining it with the kanji form of 'keikoku', 渓谷, obtains 忘却の渓谷 ("Boukyaku no Keikoku", 'Valley of Oblivion') which is the Japanese name of Oblivia. This also aligns with the JASRAC version of the title.