There are four major Sertori hunting groups in the rules but I will only talk about three because one of them is a bit of a GM secret. I find using the following people and organizations sparingly helps keep players on their toes and can turn the tables in an interesting way.
The first is a group led by Gesara of Palus in the Vaaran Kingdoms. She has the ear and loyalty of King Hanno-Kar, who believes in her campaign to purge the world of Sertori. She is fully described in the NPC section of the book. She is just a normal human with no magical abilities but what makes her dangerous is her knowledge and her network of followers. She also is protected by a couple of Krut (monsters that are particularly skilled at killing Sertori) and many ogres (who are resistant to Sertori magic). Because she is so close to the king and Sertori are actively hunted in Palus, Gesara makes a great enemy. I've expanded on her a lot in my own campaign and will be posting new ideas and twists for Gesara here in the coming months.
The second group is my personal favorite: The Monks of Isharna. They don't just hunt Sertori, they also hunt the servants of the gods (called Den Dwellers), Grim Beasts and other monstrosities (even the gods themselves). They are led by a Sertori named Isharna and he doesn't hunt them indiscriminately, he only goes after Sertori he considers a threat to humanity. Armed with a number of powerful items they exert considerable influence in Phra Goa and any Sertori passing through would be wise to be on their best behavior.
The third threat to Sertori is the city of Qam'ua. Led by a High Priestess of Sul named Dhala they believe Sertori should serve them and they use an item called the Horn of Senga to make them their slaves. The Horn enabled Dhala to create an inner circle of 12 Sertori who do her bidding. This allows her to hunt, kill and even enslave other Sertori. Qam'ua exists on an important trade road and is certainly to be avoided by Sertori who know its reputation.
These are the major Sertori hunters in the Sertorius rulebook. Like I said there is one other significant group but they are a bit secret and require a bit of rummaging in the text to find. Next time I will talk about another threat I added to my own campaign that isn't in the rule book: The Blood Cult.