Homestead Arcana

You gey this by following the main story quest. Once you have healed the 3rd Miasma scar a "lighthouse" type figure appears, which is another tree but with a bright light on its top that spins. If you get hit by this light you will lose some health until you get knocked out, so be careful here, there are places you can hide behind and wait for it to pass. The objective is to unlock all 3 of the locks in one go and then investigate the tree, where you will see a cusltscene and should get the achievement straight after this. The locks that you are looking for are not hard to find as they are huge, with one being right at the start where you first enter the area, the second is just off to the left of the first, near the "boss" tree and the last one is off to the far right. This area is easily traversible as well so just duck and cover where necessary.