When you visit Shar's Gauntlet in Act 2 you will soon run into groups of rats which you could either talk to (with a potion of Animal Talking - to find out that there is something special about them) or wait till you run into a special devil in the depths of this dungeon, who tells you about the curse, that is responsible for their strange behaviour. I won't spoil to much about it, but you can begin to kill these rats (they are not hostile per se) until you get another, last warning. If you kill the "messenger rat" as well, there is the possibility later on to get to the feet of the massive Shar-Statue, that looms over the final area in this dungeon.
When you get there you will be confronted with a massive horde of rats, that will be swarming your position. Give Shadowheart something alcoholic to drink in the inventory (best something that holds for more than 2 turns, but 2 turns were enough for me), activate Spirit Guardion (as mentioned in the other guide) and just wait until they are all killed ny this aura. It's enough if you keep your party close enough together, that the rats need to enter "the Zone" to reach your party members but you can also think about sending Shadowheart in alone. Maybe buff here to keep the concentration because some of the rats are necrotic rats and explode, which can break her concentration.
You also get a nice and handy reward for completing the quest as well.