Brutal Monsters are extra strong versions of regular monsters, most bosses are Brutal as well.
Brutal monsters can appear anywhere in the tunnel, or anywhere starting the 16.0 km mark. In dungeons after the 16.0 km mark, all monsters are brutal. Brutal monsters will never appear in the training area.
Compared to their normal counterparts at the same level they appear to have a skull in their name.
Brutal monsters' stats follow the same rules as the ones outlined in Monsters. However, they also get a 100% boost to HP, as well as a 50% boost to Power and Speed.
Brutal monsters provide the same amount of experience and gold but can drop treasure chests containing equipment.
The Avoid Risk skill will reduce the spawn rate of brutal monsters by 90%. (Doesn't apply in Dungeons)