Our questioner didn't doubt that John Milton had written those words, and he offered no argument about the truth of its content either. However, he did wonder why the verb suage isn't found in the Collegiate Dictionary. Is it really a word?

We'll answer the second question first: yes, suage is a word, or at least it was back in the 1600s when John Milton was musing about the power of words. As you might have guessed, suage has the same meaning as assuage: "to lessen the intensity of (something that pains or distresses); mitigate; appease." Since suage (and its variant swage) faded from ordinary usage by the mid-1700s, the word isn't included in modern abridged dictionaries. However, many archaic words (including suage) are found in unabridged dictionaries. Why? Because folks are still reading John Milton, and they still want to be able to look up unfamiliar words.

By the way, suage is not the only now-archaic synonym for assuage we found. Between the mid-1500s and the early 1700s, folks also used the verb lenify, a word with the same Latin ancestor as lenient. So why did John Milton select suage over lenify? Perhaps he believed suage was the more apt choice of words to ease those tumors of a troubled mind.