Clydog is the eponym of Clodock in Herefordshire, the earlier name of which, according to the Liber Landavensis, was Merthyr Clydog. Place-names in merthyr are thought to have arisen in the earliest centuries of Christianity in Britain, and might indicate that the places so named were home to early Christian burial sites. It is less certain, however, that the personal name Clydog was associated with the site from the beginning, since there is some indication that names of the type Merthyr X may have originally been formed with the simplex Merthyr only (Parsons 2013: 40–54). It is equally uncertain whether it was specifically stories about the martyrdom of a King Clydog that gave rise to the place-name Merthyr Clydog, or whether such stories were inferred from the place-name (cf. Parsons 2013: 28); indeed, both could be true to some degree. All that can be said is that, by the time that the Liber Landavensis was written, there existed a story that Merthyr Clydog had received its name from the martyrdom of a certain King Clydog, son of Clydwyn, who had been buried next to the river Monnow (though not at the exact site of his martyrdom). According to the text, Clydog had been killed by one of his men out of jealousy for a maiden, who had previously declared that she could love no man other than the virtuous King Clydog.

The text is preserved in two almost identical copies, found in the Liber Landavensis and Vespasian A. xiv. The same pattern of survival may be observed for the Lives of Dyfrig and Teilo, though in the case of Teilo the versions in the two manuscripts represent distinct recensions of the text, Vespasian A. xiv seemingly preserving the earlier of the two. Although in Vespasian A. xiv the Life of St Clydog does not occur directly adjacent to the Lives of SS. Teilo and Dyfrig, which appear consecutively, all three Lives can be seen as forming a package of material inherited by the compilers of Vespasian A. xiv from Llandaff. The rubric of the Vespasian Life of St Teilo claims that the text was redacted by Geoffrey, brother of Urban, bishop of Llandaff, while numerous features of the Lives of Dyfrig and Clydog link their redaction generally to Llandaff and specifically to the environment that witnessed the compilation of the Liber Landavensis.

There is little evidence that the extant text of Clydog’s ‘Life’ is based on any Passion of Clydog written prior to the compilation of the Liber Landavensis. Many aspects of the text link its composition with the composition of other texts in the Liber Landavensis, and the overall purpose of the text is to demonstrate the development of Merthyr Clydog from the burial place of a martyr to a possession of the church of Llandaff. First comes Clydog’s martyrdom and burial, and the establishment of an oratory at the site of his tomb ‘upon the advice of the bishop of Llandaff and the clergy’ (§1). The story of the martyrdom is told in remarkably similar terms to the story of the martyrdom of King Tewdrig of Merthyr Tewdrig, as found in one of the charters appended to the Life of St Euddogwy, which also resulted in the building of an oratory and the gifting of the territory to the bishops of Llandaff (VSOudocei(LL), §14). Following a miracle (§2) of a type found elsewhere in the Liber Landavensis, cautioning the dangers of perjury (e.g. VSOudocei(LL), §§19, 26), the brothers Llifio and Gwrfan (whose names are found elsewhere in the Liber Landavensis as those of the two bishops involved with the gifts of Llanfihangel Cwm Du) establish a church on the site of the tomb, again ‘improved by the advice and support of the bishop of Llandaff’, and the territory around the church (called Penbargod) is given to the brothers by an unnamed king of Morgannwg (§3). This section finishes with a note about the brothers’ nephew, Cinuur⁠, whose five sons divided the territory into five districts for themselves and their descendants. Presumably this note was designed as an origin legend for the territorial divisions of Penbargod at the time at which the story was composed, and reflects genuine knowledge of the contemporary circumstances of the region. The trajectory culminates in the following section (§4), where Ithel son of Morgan, king of Glywysing, offers up the whole territory of Merthyr Clydog to Llandaff. According to Wendy Davies, this gift may derive from an original charter, which she would date to c. 740 (Davies 1979: 114). However, if it does derive from some such early document, it does not necessarily follow that it was prefaced with an account of the martyrdom of Clydog and the development of his shrine much before the compilation of the Liber Landavensis.

The final two sections are perhaps the most curious. Both of them agree with the logical sequence of the text, since they both describe gifts made to the martyr Clydog and Llandaff, the implication being that Llandaff now owned the church of Clydog, as was established in §4. But neither conforms to the standard format for charters found extensively elsewhere in the Liber Landavensis (and indeed in the preceding §4). The first (§5) describes a gift of land by Ithel son of Æthelberht, ‘a certain powerful man in Ewias’, to Clydog and Llandaff, but the land it not named, nor are any witnesses listed. The second (§6) is a very short text noting the gift by the sons of Cynfleiddiau of Llech Llwyd to Clydog and Llandaff. Again, there is no witness list, but there is a short boundary clause, giving the boundaries for both Lechau Llwydion (the plural of Llech Llwyd) and Llennig (‘Little Church’). Previous commentators have treated these two sections as a single document (Davies 1979: 114–15; Davies 2003: 122–4), presumably because the first section does not name the territory that was given, but there is nothing to suggest such a link, and the donors are different in each case. It is striking that neither of these gifts is made by a member of the royal family of Gwent and Glywysing, as the majority of the Llandaff charters claim to have been, nor are kings involved with either of them at all (cf. Davies 1978: 65). One wonders whether the two sections might derive ultimately from early records kept at Merthyr Clydog. This suspicion, raised purely on internal grounds, receives support from the arrangement of the text in Vespasian A. xiv. Despite the logical progression of the text in the Liber Landavensis, as observed above, the text in Vespasian A. xiv is arranged differently, with the two gifts in §5 and §6 appearing immediately after the account of Clydog’s martyrdom in §1. It has been suggested the arrangement in Vespasian A. xiv is the more logical, because it places the post-mortem miracles together (§5 and §2, though §6 intervenes: Hughes 1980: 61; Davies 2003: 124). However, as the text currently stands in both manuscripts, the arrangement of Vespasian A. xiv is less logical, because the gifts in §5 and §6 assume that Merthyr Clydog had already been given to Llandaff, which does not occur until §4. What the arrangement in Vespasian A. xiv may in fact suggest is that §5 and §6 derive from a distinct source, which was inserted into the rest of the composition at a late stage.