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I had already mapped the entire dungeon before even getting here when I sent my familiars to explore and scout it, every cavern section, everything was now written on my Grimoire. With it open, I easily guided my entire team through it all, making sure to stop at every place with a large concentration of these Mithril Drakes and hunting every last one of them. Due to the size of the mountain, it still took us roughly an hour and a half to finally get to the Lower Section of the Mountain's caves, where on its last depths was the White Dragon.
We encountered no other new monster, but variants of the same Mithril Drakes, there were smaller, larger, quicker, and bulkier variants, but none were really qualified to be absorbed as different monsters, sadly.