In Sertorius there is a special kind of monster called a Grim Beast. This is actually an entire category of creature (belonging to the broader division of Grim). When Sertori (spell casters) misuse their magic they can transform into terrible entities ranging from something like a haunting presence to a warped being called a Grim Beast. Grim Beasts can be unique or one of several major types and they are largely twists on creatures from classical mythology and legend. Grim Beasts include Minotaurs, Harpies, Banshees, Liches, Gorgons and more.
Grim Beasts are wonderful fodder for adventure. They are tougher than many other monsters and usually can't be killed permanently without doing something to lay their spirit to rest (and this often ties in with their personal history). Killing a Grim requires a bit of investigation, a bit of combat and a lot of bravery.
When I plan an adventure for Sertorius often I will begin with a Grim Beast. For example I ran one session set in Rostanba where the Player Characters were members of the Fellowship of Promestus. I decided to make the head of the order (The Archon) a Minotaur. He had recently become one while dealing with a threat away from the city. The interesting thing about Minotaurs is people who turn into them have no idea because they retain their normal form and mind until they get angry (at which point they change into a Minotaur, go into a rage, then have no memory of the event). This worked perfectly because the adventure was a murder mystery where the Fellowship was trying to track down a killer in the city. All members of the Fellowship of Promestus take a magical Vow making it impossible for them to harm other members of the Order or to bring harm through magic to the innocent. This created a fun scenario because all the victims were guilty of something and whenever the party managed to have an encounter with the killer he couldn't harm them physically. It made it into the Sertorius rulebook as the adventure seed "A Minotaur Among Us".
Minotaurs are a strong foe to throw at a party. They look much like a traditional Minotaur but can breath fire and need to feed on humanoid flesh. The hunger is so strong that they desire nothing else once transformed. Minotaurs are incredibly strong, immune to non-magical attacks and do a tremendous amount of damage when biting. Like other Grim Beasts they also retain whatever Sertori magic they had before the transformation.
Of course Minotaurs are not the only Grim Beast. There are many: Banshees, Brain Wasps, Demagogues, Dryads, Dybbuks, Harpies, Gorgons, Liches, Marghasts, Ophidians, Sirens, Ghouls and Abominations. Grim Beasts can also be unique, in which case the GM customizes as needed based on the background of the creature.
Some Grim Beasts are tied to a particular area, but many are not. If a Sertori is powerful enough rather than become a Grim Beast he or she turns into a true Grim. Grims transcend their previous physical form and are more like forces of nature (though they sometimes do have a physical shape or an ability to manifest physically). This can be expressed in a variety of ways depending on the individual case.
True Grims are also excellent sources of adventure but take a bit more work than Grim Beasts. A Grim can be almost anything from a presence in a warped forest to an entire pocket dimension that takes weeks to explore. Each one is unique. And they are not always evil or dangerous (though certainly most are). There is one Grim in the Sertorius setting called the Tungat Oasis. This is an entity in a desert region that produces water to create a habitable oasis and a city has emerged around it. The locals revere the being, who appears to them as a vast head inside the waters. When they entertain it with music or stories, the waters stay pure and clean, if they allow it grow bored, the waters turn bad. This is an example of one of the less malicious Grims in Gamandria. However it is an exception.
A darker example is the Foothills of Yaum, which is an enormous stretch of hills and woodland infused with the Spirit of the great Sertori Yaum, who was famous for his splintering magic. Yaum can manifest wherever he wishes inside the Foothills and looks dwarfish but when harmed he splinters, multiplying further and further with each wound. He also harbors dozens of personalities and anyone encountering him can never be sure which one they will meet. The land itself is inviting and feels like a nice place to settle. Many villages have cropped up here and shepherds are a common sight in the hills. But anyone taking a closer look at the locals can see they share familiar features, their faces (even though they may be of different races or genders) all look hauntingly similar. This is because anyone remaining in the Foothills too long starts to transform until they resemble Yaum. The land here is also affected by Yaum's magic, which is fueled by hate and rage. This affects anyone in the foothills, even if they are just passing through, requiring regular Resolve Checks to avoid flying into violent rages.
These are just a couple of instances from the book. In all Chapter Six of the Sertorius Rulebook describes thirteen Grims. These are meant to serve as models to help GMs create their own.
Grims and Grim Beasts are important features of the Sertorius setting. They are easy to overlook but absolutely worth checking out if you haven't yet. I encourage anyone running Sertorius to check out Chapter Six: Afflictions and Grims for an explanation of the process. Pages 198 to 208 gives descriptions of the official Grims in Gamandria. Chapter Eleven pages 379 to 388 contains the Grim Beast entries.