This trophy is for finding the covenant of the Rat King in the Graves of Saints area. This is accessed by dropping down the large well in the middle of Majula. You can wait until you find Laddersmith Gilligan and get him to set up a ladder or you can buy the cats ring from the cat merchant in Majula to allow you to jump down. Bear in mind this is still tricky and requires a good level of health.
Once you are in this area you will need to find the second bonfire before a fog gate where you will fight the mini-boss fight with a bunch of rats. You need to look out for the rat with the spiky hairdo once you have killed enough regular rats for him to appear.
Once killed you can make your way as shown by the video to the Rat King.