potion seller has a potion you can buy after beating the lich boss and touching the vat
one for going back to normal
one for becoming slime
what potion shop the one i know only sells the growth potion and when i did beat the lich and go into the vat it never sold anything like that
talking instead of trading with the potion selling they will mention the lich and have new potions for
deliching your self or making your self a slime again