Even from below, Duke could sense the awe-inspiring scale of the structure, built entirely from Seaheart Iron and other Magic Metals.
Its underside was just as smooth and seamless, radiating a powerful energy field with almost no visible weaknesses or gaps.
Duke placed a hand against the outer wall of the underground vault and clearly felt energy coursing through it.
He could also tell that the vault's outer wall was indeed made of Seaheart Iron.
Duke then began using the Yellow Sand Crystal to slowly absorb the Seaheart Iron within the wall. The process was extremely slow.
He deliberately kept it that way. Absorbing it too quickly might cause a change that the people above would notice.
As he worked, Duke watched the underground vault closely. Nothing much changed.
The Yellow Sand Crystal's absorption was remarkable. It did not seem to rely on any spell; it was more like a physical process.
To the underground vault, it was as though corrosion of its outer wall had simply been accelerated.
Normally, a structure made from Magic Metals would take many, many years to show any obvious signs of corrosion.
But the Yellow Sand Crystal was producing a similar effect now. The vault wall before Duke gradually grew thinner.
Time slipped by in the silent depths underground.
Like the most patient of craftsmen, Duke controlled the Yellow Sand Crystal, eating away at the seemingly indestructible Seaheart Iron wall bit by bit.
The work was painstakingly slow and demanded his full attention. He could not afford to rush; absorbing it any faster might alert the wizards above, who were trying to breach the main entrance.
At last, through his relentless efforts, a small, irregular opening appeared in the outer wall at the bottom of the vault's deepest level. It was barely wide enough