One of the mainline story quests gives you magic seeds, you plant these magic seeds in your fort, and you get a luck tree sapling. You feed the sapling items, and it magically grows the tree. The tree has 5 levels, the fifth and final level is the Ancient Luck Tree, which gives you +5% gear luck when hunting (higher chance for legendaries to drop).
This achievement appears to be buggy, as both I and my co-op partner got our trees to level 5, this achievement popped for him and then relocked itself, but never popped for me. I've gone to my tree many times to try and trigger the achievement, but nothing.
This achievement was fixed in the latest update. If you grew your ancient luck tree prior to the update, simply pick the tree up, move it and put it down, and the achievement should pop. If this doesn't work, try putting it back into your inventory before placing it again.