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After The Spider came into possession of a young beast whelp, Gaelin-4 learned of its existence and asked for it as payment for recapturing an escaped Cabal Wrathborn. After completing his mission, Gaelin informed Vynriss that he would be naming the pup Castus.
After growing older and being trained by Gaelin, Castus was used as a tracker by Gaelin. While on a mission in the Venus jungle with Katabasis, Castus led the way towards their target. The two Hunters discussed the beast, with Katabasis noting that the war beasts name was drawn from one of the Spider's history books which Gaelin often availed himself of.
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