Complex sentence: The Toadstone (not unlike the batrachite) is a mythical stone or gem thought to be found in, or produced by, a toad, and is supposed to be an antidote to poison.
Split into simpler sentences: The Toadstone (not unlike the batrachite) is a mythical stone or gem supposed to be found in, or produced by, a toad. It is supposed to be an antidote to poison.