Draugr origins

From the runestone:

Long ages ago, the world of Valheim was home to a race of proud and noble people. They built great towers that touched the clouds and delved deep into the earth for precious metals. But their pride was their undoing. They challenged the very gods and went to war against the Aesir and Vanir, destroying themselves completely. Great Odin leveled their cities and Loki consumed their mines in fire. Mighty Thor broke down their towers and Freya sowed their fields with salt tears. Now they are sunk many hundred of years deep. No song sing of their stories, and the earth has claimed their cities. But pride cannot be killed entirely and the warriors who fought in that final battle will not surrender until their bodies are dust and the dust long gone. They return as Draugr, unholy walkers in ancient armour, creatures of rust and despair. Break them, bury them, let them know they are dead.

Odin never intends for you to make it to Valhalla – if he did you'd have made it there from your actions in your previous life. You don't remember your previous life though, and so it seems pretty likely to me that you did something dishonourable and ended up sent to Valheim instead to clean up Odin's little mess.

The Forsaken gathering power isn't something new though – it's been happening over and over again in a cycle. They gather power, Odin sends in a bunch of powerful warriors who did something dishonourable and were denied Valhalla to clean it up. Then when they've done so (and built cities in the Meadows, made great big towers like we see on this sub, delved deep into the earth for precious metals so they can collapse ore veins), they go to Odin and say 'can we go to Valhalla now?' – and Odin says no.

So since they've had a good run at the Forsaken and are feeling pretty strong, they war on the Aesir and Vanir, get their arses whooped, and they all die. Ready to be the Draugr for the next batch of suckers Odin calls in the next time the Forsaken are gathering power.