Molrat is an underground realm, once again, not unlike the Underdark of prime worlds. And of course, with a perverted twist that only the Abyss can provide.
The layer is eternally dark and all the fiends there have become haters of light. All have developed some form of blindsight or scent ability to compensate. In fact, Darkvision do not work in Molrat.
The layer itself seems to devour light, whether it is from mundane or magical sources. They work initially but become dimmer and dimmer until they are extinguished totally. 
It is not known what demon lords might rule this layer, but undoubtedly it appeals to those who prefer darkness.

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