"What is this thing?"
This disambiguation page lists articles associated with the same title. If a link led you here, you may wish to change that link to point directly to the intended article.
Vs. Arachnus may refer to one of two boss themes for Arachnus. You may be looking for:
VS. Marukara, the Arachnus theme from Metroid Fusion
Boss Arachnus, the Arachnus theme from Samus Returns