Emperor Ing Spider is the name of the theme that plays when fighting the final form of the Emperor Ing in Metroid Prime 2: Echoes. Its name comes from the Soundtrack Gallery in Metroid Prime Trilogy, where it can be purchased for one Silver Credit. It was originally composed by Kenji Yamamoto.
Emperor Ing Spider opens with part of the melody from Metroid Prime 2: Echoes Theme, which in turn was remixed from the melody of Metroid Prime Theme. This seemingly mirrors how the Metroid Prime Battle music opens with the melody of the main title theme.
The rest of the song features relentless percussion beats, a dramatic low choir, and an intense melody played on a flute-like synth instrument, making for a climatic boss theme. The melody seems to take inspiration from classic sci-fi music and exotic Middle Eastern music, like a UFO B-movie version of a snake charmer's flute song. Part of the melody sounds somewhat similar to the low choir from the Sanctuary Fortress theme.
It can be heard here: