The Bone Breakers have managed to gather only limited information about the Horned Rat.
Appleby, the hedge wizard in Fort Sturm, told the heroes that the Horned Rat is a god or demon lord worshipped by the skaven, who had unsuccessfully attempted to summon him to Ahltorass in the past. The skaven sometimes eat the dead after battle and sacrifice prized parts to the Horned Rat. But their priests are few and they cannot heal.
Skaven rally behind this banner:
From the tattoos on the dismembered Skaven Runner found on the muddy road outside of Ashenport the heroes discovered that the skaven have seized the Weeping Gates and intend to attack the Stonelands on a large scale in the spring. They offer the dwarf kingdoms, including Darg’s home of Dormund, to King Bronnor and invite him to move his armies through the Weeping Gate.
From the crystal ball of the Vistani fortune-teller Illyra, Mesmer learned that the Weeping Gate is a waterfall in the mountains. When Mesmer asked about asked about the Horned Rat he saw images of a torchlit cavern with a rearing, huge-sized bull-horned rat demon or rat god surrounded by a massive teeming army of skaven of many sorts.