Agder, south of Norway, before Harald Granraud's death

......I suppose you're talking about the alleged grandfather of Halfdan the Black by mother side, father of Harald Fairhair in later historical traditions, mentioned in Chap. 48 of Ynglinga saga, right?

If my knowledge is correct, his name firstly appeared in Ynglinga saga (13th century), not before. It is true that controversial Ynglingatal (that I regard it as a later literary fake) and middle-12th century Historia Norwegie ('History of Norway') mention the assassination of Gudrod (Guthröth), instigated by his wife Ása, alleged daughter of Harald Granraud, but none of these sources allude to her origin.

As I suggested before in Did Harald Fine/Fairhair actually exist?, the majority of scholars now regard any ruler narrated in later saga traditions prior to Harald Fairhair primarily as legendary (that is to say, not purely historical) figures (Cf. Dillmann 2008).

So, I cannot and don't want to discuss the historicity of alleged Harald Granraud and his petty kingdom in Agder, southern Norway, here. While I'm not really good at the calculation of date, however, I can instead suggest some possible political and social circumstances in 8th century now Southern Norway.

The population size

Before the beginning of so-called Viking Age, it was likely that the demography of now Norway was probably a bit less than 40% of those of medieval zenith in ca. 1300, about a half million (check my previous post in Did the Nordic countries use to have a comparatively larger population back at the time of the vikings? If not how were they so often able to raid Britain? In short, the total population size would have been ca. 200,000 or less.

In high medieval Older Gulathing law (the middle of the 13th century?), farmers in Agder were stipulated to provide 16 of twenty-five bencher warships (a bit larger size than the normal twenty bencher warship) (Chapter 315, Simensen trans. 2021: 210). The number of fully conscripted farmers from Agder would have occupied 6.4% of those from all the Norway, then, in my very rough calculation, though the number of ship might in fact not have be based on the actual ratio of the coastal population by region (fylki), but rather the agreement of the king and people in medieval period, as suggested in Chap. 297 of the same law book (Simensen trans. 2021: 199). Anyway, if we assume the basic settlement patter did not drastically change ca. 8th century to the 13th century, the (very, very) rough estimated population of whole Agder (both eastern and western) would have been a bit more than ten thousands (less than ca. 12,800).

It was probably another matter, however, whether or to what extent these peoples in now Agder in the 8th century were integrated in the emerging territorial lordship of chieftains.

Then, much more further difficult part comes: The number of local power centers (local chieftaindom) in 8th century Agder. It was known that a chieftain around Lindesnes (near Kristiansand) in Agder built Spangereid Canal (linked to the official site of Visit Norway) in the 8th century, so the power concentration into somewhat larger territorial lordship also occurred there in course of the 8th century, but he didn't probably unite all the coastal settlements within the region under single rulership, otherwise his name [Harald Granraud? Possibly?] would have been remembered as a pioneering predecessor of another Harald ('Fairhair') in later historical tradition. This was probably exactly 'historical' Harald fairhair did for coastal Rogaland and middle to southern Hordaland regions more than a century after, as some researchers suggest.

Alternatively, Norwegian archaeologists seem to suppose the existence of at least two possible political centers respectively in Lista (western Agder) and in Fjære (eastern Agder) within Agder from later Roman to pre-Viking age, mainly based on the distribution of burial mounds and luxury objects (Solberg 2003: 166f.).

[Added]: Solbergs charts at least 9 such kind of possible larger-scale power center located in Western and SW coastal Norway from Nordfjord to Grenland (Solberg 2003: 166).

I also wonder whether my previous post in In what ways were pre-Viking Scandinavians (6th, 7th, 8th centuries) culturally distinct from their 9th century descendants? In the areas of language, religion, economics, military tactics, technology, social structure, etc. might be of any use.

I have difficulty in presenting you with any (at least somewhat) reliable introduction of this period/ region in English, but the English dubbed version of the introductory of Nordvegen Historical Center (though in Rogaland) might also be helpful: https://www.youtube.com/watch?v=vTj3XAspApw

References:

• Simensen, Erik (ed. & trans.). The Older Gulathing Law. London: Routledge. 2021.

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• Dillmann, Francois-Xavier. 'Omkring Halvdan Svartes gravfeld'. In: Høvdingen: Om Snorre Sturlasons liv og virke, red. John O. Askedal & Klaus J. Myrvoll, ss. 73-86. Oslo: Vidar Forlaget, 2008.
• Solberg, Bergljot. Jernalderen in Norge: 500 før Kristus til 1030 etter Kristus. 2. oppl. Oslo: Cappelen Akademisk Forlag, 2003.