On this page of the Assassin's Creed Valhalla Wrath of the Druids game guide, you will find a detailed walkthrough of The Ivory Post quest.
By rebuilding the trade post in Port Auley, you will gain access to Ivory - one of the main luxury goods in the Wrath of the Druids expansion pack.
Reward for completing the mission: 1,700 XP.
Investigate the Trade Post
Once you have cleared the trade post, read the note lying on the wooden box in one of the side rooms in the location pictured in the image above. You will learn where to look for the document you'll need to rebuild this trade post.
Obtain the deed
Head to the location indicated on the world map and use Odin's Sight to locate the chest containing the document. At this point, it's a good idea to use the explosive arrow or oil jar and blow up the debris seen in the image above to get to the chest immediately.
An alternative route leads through a complex series of caves - the entrance is at a lower level.
The chest is protected by one Elite Druid, but that shouldn't be much of a problem. Take the document and return to the trade post.
Restore the Trade Post
Return to the trade post in Port Auley, interact with the wooden board and confirm the construction of the new building. This will complete The Ivory Post quest and you will receive 1,700 XP as a reward.