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In YGGDRASIL, Grenbera Swarmlords were one of the monsters that spawned around the swamp area where the Great Tomb of Nazarick was located originally. It guards the unconquered Great Tomb of Nazarick from enemy players and NPCs if caught on sight trying to trespass into their territorial ground before reaching the tomb.
Grenbera Swarmlords are monsters that have adapted to living in a swamp-like environment. In YGGDRASIL, their level can reach as high as level 78.