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Sylas walked out of the cabin with the Basilisk King by his side. Now that its stats had improved to this extent, it could keep up, and it had its value as a companion outside of their fusion.
Plus, it was clear that the way to help the Basilisk King fully awaken its pride was through combat. So, he let the Basilisk King take the vanguard as they walked back toward the Portal. In the meantime, he took out an orb.
This orb was the final mammoth's Gene Core. The Basilisk King hadn't ended up swallowing it, and even if it did, judging by how things were going, it wouldn't level up regardless. There was definitely a special condition needed to help the Basilisk King level up, but for now, maybe it was best to wait on raising its Grade to FFF+ first.