D&Dtools1

Your body bloats and writhes until eight massive spider legs burst through your flesh. As the transformation finishes, your skin hardens into a dark exoskeleton.

As lesser spider form, except you take on the form and abilities of a Large fiendish monstrous spider (MM 289). You are treated as having the spider's Hit Dice (4) for the purpose of adjudicating the special abilities gained from the fiendish template.