"See the old Dragon from his throne
Sink with enormous ruin down!"
The attribution Cordell provides is merely 'Hymn'. In years past I might have never located the hymn from which these lines were taken, but, thanks to the power of the Internet (I simply typed in, as a quotation, the first line of the epigraph into Google search), I discovered that one source for the hymn is the third volume of the Works of the Rev Phillip Doddridge.
The Rev Doddridge, who wrote the hymn based on the passage from Revelation 12.11 ('But they have conquered him [i.e., the Devil, the dragon] by the blood of the Lamb and by the word of their testimony, for they did not cling to life even in the face of death.'). Doddridge's work The Rise and Progress of Religion in the Soul influenced William Wilberforce to become a Christian (so Wikipedia). Doddridge himself is a somewhat obscure figure. His online presence is largely limited to his Wikipedia article and some online hymn sites.
I am not sure where Cordell discovered these lines of the hymn; I should ask him one of these days.
What the purpose of text from the hymn is as an epigraph in a D&D adventure module, I shall not say, but I was inspired to look for it based on some reading I am doing while writing a meditation on Sauron, the chief villain of The Lord of the Rings for The Marginal Virtues. Sauron's name derives from the Greek word 'saura' (whence also 'dinosaur'), as I was reminded while re-reading Planet Narnia (by Dr Michael Ward), in which the Solar imagery of The Voyage of the 'Dawn Treader' is discussed and related to the European Solar deity par excellence, Apollo, in his role as Apollo Sauroctonus, Apollo the Lizard-Slayer.
There is nothing serpentine about Sauron, of course; but his name and character are reminiscent of the Dragon of Revelation (he probably has more of Milton's Satan about him, I might say), and the first two lines of the hymn which Cordell quotes stand in as an apt summary of the destruction of Barad-dûr when the Ring falls into the fires of Mount Doom.
One of the enduring gifts of literature, I think, is its ability to bring the common themes underlying life to light in fresh and imaginative ways. Doddridge's hymn, the war against the Dragon in Revelation, Cordell's D&D adventure, The Lord of the Rings, and The Voyage of the 'Dawn Treader' all illuminate (the which is a lively metaphor in the case of the last work mentioned), in their own fashion, an aspect of human life which can be experienced (by transposition) on many levels. 'Here there be Dragons' - yet, here also, are they who provide us with the ways and means to defeat them.