Su Yang's special training did not take place inside the World Melting Furnace, but rather in the Storm Valley.
The four Beigong War Gods received Divine Artifacts from the Feather Spirit Palace, and although the three Crystal Skulls did not obtain any Divine Artifact, the gourds atop their heads were not inferior to those artifacts.
Because of this, these individuals have immense potential. Once their potential is fully unleashed, their strength will increase dramatically.
The Storm Valley was chosen for its ability to maximize the unleashing of a person's potential.
Of course, another reason is that the formidable power within the Storm Valley can cloak the aura of the Divine Artifacts.
Before having sufficient strength, the fact that the Human World possesses these four Divine Artifacts must remain undisclosed. Otherwise, the other Eight Realms would certainly immediately attack the Human World, leaving it no opportunity for development.
