The Maw Threatens All

Recent attacks at The Maw, just outside the capital of Hope, leave tensions high.
The Knights of the Eclipse have ever-vigilantly watched over the doorway to the Below and the city's populace are not unused to tidings of vicious attacks and acts of valour.
Recently though, perceptive townsfolk have noticed that there have been many more men returning on stretchers in the last few months compared to recent years. Fearmongering and propaganda arose, claiming that something had happened at the gates which threatened the city's safety.
In an attempt to address the rumours, Emperor Davistan XIV himself made a statement to the public. In quite a long speech, he assured the people that while the fighting was tough the Empire's elite had everything under control and there was nothing to worry about.
The Emperor, however, made a fatal error in his speech. He told a story of a great battle that had happened at the ancient doorway recently - a story which seemed strangely similar to one of the great stories of yore - and the populace have picked up on the Emperor's deceit.
What horrors could be transpiring at the ancient battleground that would threaten to overwhelm the Empire's elite defenders and force the Emperor himself to intervene and lie to his people?