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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.