Eliot originally planned to use the following quotation from Joseph Conrad's novella Heart of Darkness as the epigraph for The Waste Land. Marlow, the tale's narrator, has travelled up the Congo River to meet the trader Mr. Kurtz and is now travelling downstream with the dying Kurtz.
Did he live his life again in every detail of desire, temptation, and surrender during that supreme moment of complete knowledge? He cried in a whisper at some image, at some vision, -- he cried out twice, a cry that was no more than a breath --
'The horror! The horror!'
However, during the poem's editing Ezra Pound told Eliot in a letter that he did not think that a quote from Conrad had enough weight. Eliot wrote back that the epigraph was the most appropriate he could find and it was elucidative. Although Pound was ready to concede, Eliot replaced the epigraph anyway using one from The Satyricon.
This site has an excerpt from the section of Heart of Darkness where Kurtz dies.
Citation: "The Facsimile" pp. 2-3, 125