As its name indicates, this track serves as the boss theme for Amorbis, fought in the Dark Agon Wastes. In New Play Control! Metroid Prime 2: Dark Echoes and Metroid Prime Trilogy, the Amorbis theme is available for purchase in the Soundtrack Gallery, costing a single Silver Credit.
Amorbis is vaguely reminiscent of the Parasite Queen boss theme from the previous game, featuring similar relentless percussion and dissonant high-pitched synths; a pair of particularly harsh-sounding chords near the end of Amorbis sound very much like part of Parasite Queen's melody. However, Amorbis has a distinctly different melody; it is largely sung by a deep male choir, giving the song a much "weightier" feel as representing a dark and powerful behemoth, in contrast with the frightening tension of Parasite Queen. Therefore, its more subdued tone makes this song feel slower and heavier, despite actually having a faster tempo than Parasite Queen. Amorbis also has a similar beat to the Chykka boss themes, musically uniting the bug-like bosses of Dark Aether.
The theme can be heard here: