Answer:
The corporeal beast is currently the secound strongest monster in runescape as of 24/08/2011. It is members only to fight and can only be fought once the quest summer's end is completed. The corporeal beast is mostly killed for its sigil drops. these are spectral, arcane, elysian and divine. A full list of its drops may be found at this link http://runescape.wikia.com/wiki/Corporeal_Beast as well as a guide for it killing the corporeal beast.