Those traces of corruption could not have formed overnight. They were most likely the "nursery" where the "Counter Tide" born in this tower had first been incubated, or remnants left behind after the Counter Tide's activities. According to the intelligence provided by Tarlonnd, those traces could very likely be dated back to ancient times—to a million years ago, on the eve of the Counter Tide Empire's destruction by the dragons.
But when had that rift appeared?
Gawain did not know. Perhaps no one did. The rift clearly existed in a state of "noninterference" with the entire hall, passing through the material world like a phantom, cutting through steel and air alike. Gawain could not judge from the surrounding traces exactly how long it had remained here. It might even have existed before this tower was built, or it might have silently formed only an hour ago... As a "phantom" that could not be observed through conventional means and did not interact with its material surroundings after appearing, it could have emerged at any time.
Yet Gawain had an instinctive guess. He believed that thing had hung above the hall for many years, and that... it had become a breach through which something escaped.
There was no divine response anywhere in the tower, and they had found no trace of Mental Corruption throughout their exploration. That alone was an unsettling sign.
After all, Modir had been contaminated by the Counter Tide mere moments after entering this hall. He had begun obsessively absorbing and worshipping ancient knowledge. If not for the Dragon God's "interventionary protection" outside the tower, he likely would not have regained consciousness until he had been completely transformed into a Counter Tide believer.
Yet they had explored this hall for so long without showing any signs of Mental Corruption.