The main reason is that for the Rat Catcher quest, we have to enter the well located near the western gate in Cassardis, a well which is closed; however if we finish Call of the Arisen, and we return to Cassardis, we will advance the games main story-line and we will trigger a cutscene.
After the cutscene showing a strange and blind monk, if we check the well, we can see that it was unlocked.
Slay 7 mice
“If there is one thing I cannot abide, its mice! Even just to think of them makes me ill.
The other day I took the number of rodents in my home and tabulated their numbers in all the realm .I nearly fainted. I any case, I want them dead! As many as you can manage.”
The goal of the Rat Catcher quest is clear enough. We have to descend the well and we must kill 7 mice.
These creatures are easy to find, since the underground tunnels are filled with them.
They wont attack us and to complete the quest we wont have to travel back to the city.
After we kill seven mice, the Rat Catcher quest in Dragons Dogma is completed.
Note: Before descending the well, it is a good idea to talk to Poll, a soldier standing near the entrance, which has a new quest for us, called Deep Trouble.