The Forever Tree is also know as the Man-Shik, which guards the entrance to see Bon-Hwa, who grants the titles and/or rewards of Enchanted, Il San, Ee San, and Sam San.
It is located at coordinates 19,91 in the Wilderness.
If you ever see a sage saying, "Looking for a group to take down the tree", they are referring to the Forever Tree. It has a very great number of hit points, and killing it can take a while. There are also Man-Shik Crows around, which can blind you, and hit for a fair amount of damage if you don't have proper protection (and ASV).
The Tree respawns quite often, so once it's dead you have to be very quick in going by it to get to Bon-Hwa's cave. As far as I know, it only hits from behind. So, if you can get an Il San Rogue to Dash behind it, then go Invisible with a Polearm, you can do great damage.