Final answer:
The author who believes mystical visions inspired his poetry was Samuel Taylor Coleridge, exemplified in works like 'Kubla Khan', conceived during an opium-induced dream.
Step-by-step explanation:
The author who claimed that mystical visions were the source of his poetic inspiration was Samuel Taylor Coleridge. Coleridge, along with William Wordsworth, were key figures in the Romantic poetry movement, often seeking inspiration in the sublime and transcendent aspects of nature and the imagination. In particular, Coleridge's work 'Kubla Khan' is famously cited as being conceived during an opium-induced dream, embodying the mystical and visionary qualities that he claimed influenced his poetry.