Cyfoesi Myrddin a Gwenddydd ei Chwaer ("The Conversation of Myrddin and His Sister Gwenddydd") is an anonymous Middle Welsh poem of uncertain date consisting of 136 stanzas, mostly in englyn form. Myrddin, the legendary 6th-century North British bard and warrior, is depicted as being encouraged by his sister Gwenddydd to utter a series of prophecies detailing the future history of the kings of Gwynedd, leading up to an apocalyptic ending. The mood of the poem has been described as "one of despair and of loss of faith and trust in this world".