As you should have realized, this is a game set in an alternate history where Norse colonies in North America survived and grew over several centuries. Just for fun, most of the other pages are written as if this alternate history were true.
In the real world, the Norse definitely settled in Greenland and almost undoubtably did visit North America, but any settlements there died out. There are extensive archeological sites in Greenland showing the Norse colony there, and there is a debated site in Newfoundland at L'Anse-aux-Meadows. The more important evidence is the Icelandic sagas written down 3 or 4 generations later, which tell about founding of the Greenland colony and attempted settlements in "Vinland". The Icelandic sagas are outstanding not only as a view into history, but as literature in their own right. Iceland developed a fairly unique literary culture within a century after this time.
Below is a brief overview of the divergence; discussion of the real-world history of Iceland, Greenland, and Vinland; and a list of sources used.
For the full story of real-world history, see the sources list below. For purposes of this history, there are a few important points I should make:
The divergence point for my alternate history is the voyage of Bjarni Herjolfsson in 985 or 986. In real-world sagas, he sighted the western lands but since his supplies were intact and he was determined to reach Greenland, he never landed there. In my alternate history (cf. Vinland History), his ship was damaged in a storm and they happened on land just in time to save them. There he found wood to repair his ship and went back to Greenland.
This one change then lead a curious sequence of events:
Iceland also remained non-Christian for longer than the rest of the Norsemen. From the start in 860, it remained heathen for some time. The Norwiegen king ??? sent missionaries there with mostly poor results. For example, a missionary named Thrangband went in 983 to Iceland. He apparently spoke well to several godi, but a number of Icelanders mocked his religion. In response, he killed two people and then fled the country. Undaunted, ??? sent two more distinguished men in 999 to Iceland. Meanwhile, he held five (?) other visitors as hostages during this time. Read the Laxdaela saga for the account of Kjartan and the other hostages at this time.
The latter missionaries, ??? the Wise and ???, spoke before the Althing in 1000. The majority of Icelanders at that time were still heathen, but the Christian minority resulted in much feuding and division. Persuaded that this would restore order, the lawspeaker proposed to adopt Christianity and a vote confirmed this. At first this was purely lip service, for example saying that heathen sacrifices could still be done in private. However, after many years it soon became the norm.
While Iceland was nominally Christianized and democratic, it was torn by regular blood feuds and strife. It was difficult to keep civil order with their political system. In addition, Iceland traded and intermarried with the mainland, resulting in allies of the Norweigen king often coming into power. Still, it remained independent and democratic until 1262 when they were persuaded to turn allegiance over to the King of Norway. The democratic rule was a proud one, but the bloody strife was too often uncontrollable.
Around 980 A.D., Icelanders discovered in Greenland and began a settlement there, and also sighted lands to the west. The Greenland settlement lasted for several centuries and had a peak population of around 3000 people. While not massive, it apparently did reasonably well for itself.
The sagas tell of both Lief's and one other attempted settlement in the western lands which Lief dubbed "Vinland" for the grapes (or simply berries) which grew there. They reported the land was rich with resources. Both settlers had violent encounters with the native "skraelingar" and left after two or three years. The latter settlers started some trade with the natives, but relations broke down. Other settlers were likely discouraged by the remoteness and possible violence from the natives.