This creature is called the "Merfolk Voodoo Ceremony". Its level is not high, only between 5 to 7, and it has a pitifully low HP, mostly ranging from five to six hundred points. However, it is the true sentient creature among the Merfolk. Not only can it control other Merfolk's actions to a certain degree, but it also possesses a very tricky ability: spreading plague!
This Merfolk mage can bless other Merfolk with a plague-like mutter type spell. This plague is harmless to the Merfolk, and it doesn't usually break out. But once someone kills a Merfolk carrying the plague, they will contract the plague, which will continuously decrease their HP at the rate of 5 points per second for 60 seconds.
Moreover, this plague can infect others within a range of two meters, turning them into another source of infection, thus continuing the cross-contamination process...