Complex sentence: The epithalamus is a (dorsal) posterior segment of the diencephalon (a segment in the middle of the brain also containing the hypothalamus and the thalamus) which includes the habenula and their interconnecting fibers the habenular commissure, the stria medullaris and the pineal body.
Split into simpler sentences: The epithalamus is a (dorsal) posterior segment of the diencephalon (a segment in the middle of the brain also containing the hypothalamus and the thalamus). THe epithalmus includes the habenula and their interconnecting fibers the habenular commissure, the stria medullaris and the pineal body.