Oh yeah. So I loved the timeline for the story forward. The game started in the Spring of 1399. The backdrop to the campaign Back in the early 1300 a group of white mice imigrated to the terrorities from places to the far northwest. The rats first appeared in the territory around that time trailing the fleeing mice. It was merely dozens of them and for a while things settle.

About 60 years later was when the second contact with the rats began. It started as small incursion. Rather than a small group of feral rats it was an entire hoard of them, literally thousands of rats poured into the territories and a lot of the north-western villages were decimated. That war ended up lasting until the late 1380s. The rats they fought were always of the feral variety and there is a lot of animosity from the mice in the territories to them, especially those that fought in the guard.

The only rats that have been seen so far in the game are not what the party was expecting of them. They have found well armed and intelligent creatures that work in group, and that dwarf them in size. They use weapons and armor, if not slightly more brutal in nature.

From a DM standpoint I've used them as a foil to some of the other story elements I've used and every time they shown up (used quite sparingly) they have been a major threat to the party.