The opening cinematics of Dark Souls 3 had a vague introduction, but the plot was easy to understand. The ancient Lords of Cinder woke up from their tombs since they shouldered the duty to rekindle the fire.
However, these Lords of Cinder didn't want to fulfil their duties. At this moment, the accursed unkindled who couldn't become Lords of Cinder woke up from deep sleep and came out from the tombs of ashes, following the route that the Lords of Cinder took. They must return these lords to their thrones.
This was the basic storyline of Dark Souls 3.
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As for the destiny of the Lords of Cinder and why they didn't want to take the glorious thrones of lords, it was easy to understand when the players saw the words 'cinder' and 'link the fire'. They would have to burn themselves up to rekindle the fire.
The origin of linking the fire could be traced back to the very ancient times (as shown in Dark Souls 1).
