The Three Sisters - is a set of mountains, which locates The Artifact at the top of it. It served as the spawn zone of The Artifact server before it was reset, and now become Loka.
Spawn zone - is a place where players spawned upon dying. It is a protected region in which Slicers can't hurt each other or be killed by mobs. You can't destroy or build blocks there and you also can't place fluids in there, but you can open chests, use dispensers and doors.
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The Artifact - is a mystical entity that is located on top of the three sisters. It is said to be source of spawn zone's protection, being something of a heart to the world.