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To earn this achievement, you need to create a piece of living floor outside of the Living Caves. To do this, lure a Rock Troll out of the land and then kill it with an Orb of Flash - instead of leaving behind a dead troll, it leaves behind a living floor!
(The easiest way to do this is by going to the crossroads, as orbs of flash will appear there as long as you found 10 treasure in the Icy Land, then you can look for a Living Caves land from there)

Once you've got the living floor, you'll need to lure out a few more Rock Trolls and kill them on tiles adjacent to the living floor in order to coax it into becoming a living wall. Once you kill enough to make the tile flip into a living wall, this achievement will pop.