This page of the Assassin's Creed Valhalla guide has information about the historical realities in which the game is set. You will find out, among other things, when the game is set, is it historically accurate, and who Alfred The Great is.
Place of action
Assassin's Creed Valhalla takes place in the 9th century AD. This is a period of intense Viking expansion in Europe. Within 100 years, Scandinavian warriors have plundered places like London, Paris, and Hamburg. You play as, Eivor, a leader of one of the clans who has led their people to the shores of the British Isles. Upon reaching them, the Vikings find themselves in the middle of a power struggle.
The developers have put a lot of emphasis to make the game historically accurate. Vikings in the game wear clothes stylistically similar to those worn by representatives of that nation. Despite this, some of the characters have helmets with horns, which isn't historically accurate.
The game also lets you control a boat - Drakkar. This type of boat was indeed used by the Vikings. They were intended for sea and river navigation.
The series' tradition is that it features many historical figures. Throughout the Assassin's Creed series, players have encountered characters such as Caesar Borgia, Leonardo da Vinci, Sultan Suleiman, and Benjamin Franklin.
This part of the series also features historical characters. At the moment, we know one of them. It is King Alfred The Great of Wessex. He is considered one of the most prominent British rulers of the time. He was mainly famous for successfully repelling Viking attacks. In AC Valhalla, he serves as the antagonist fighting your clan.
However, it is likely that many other historical figures living in the second half of the 9th century will also appear in AC Valhalla.