Death

 is Track 26 on the Xenosaga THE ANIMATION Original Soundtrack. It plays in the following scenes:
 * Allen explains Shion's reluctance to accelerate the KOS-MOS research to a new researcher in Episode 1.
 * Virgil remembers Hornst and Lester after picking up a gun in Episode 7.
 * Shion sees a memory of her younger self in the Subconscious Domain in Episode 8.