In this version, Williams was travelling to Pontrhydyfen and composing a new missionary hymn as part of a commission from Selina Hastings, Countess of Huntingdon.

Upon passing the ruins of an old Cistercian abbey and seeing the mountains he would have to cross on a stormy night, the view inspired him to write "O'er the Gloomy Hills of Darkness" for the Countess.

[11] When the hymn became established in the United States, slave owners changed the line referencing "Indian" and "negro" to "Let the dark benighted pagan".

The abolitionist Ebenezer Davies claimed “The altered reading, I learned, prevails universally in America, except in the original version used by the Welsh congregations.

[12] With its original words, the hymn was understood to speak to the experience of slavery, even if in an evangelising frame, and Pantycelyn had previously written critically of the transatlantic slave trade.

The reason published in the Baptist Magazine stated "But the verses are too rude and unfinished to be generally accepted in modern day".

[2] Jeffrey Richards states that "William Williams's O'er the Gloomy Hills of Darkness of 1772 (sung variously to Thomas Clark's Calcutta, by Baptists; to Henry Gauntlett's Triumph, in The Scottish Hymnal; to Edwin Moss's Ulpha, in the 1982 Presbyterian Church Praise) did not make it into Hymns Ancient and Modern, but it was in Bickersteth's Christian Psalmody in 1833 and was still to be found complete in the 1933 Baptist Hymnal … This very much set the tone for missionary hymns.

"O'er the Gloomy Hills of Darkness" inspired the founder of Methodism, Charles Wesley, to write "Sun of Unclouded Righteousness" for missionaries working with "Mahometans".

[16] The lyrics of "O'er the Gloomy Hills of Darkness" as published in Gloria in Excelsis: or hymns of praise to God and the lamb in 1772.