When Morgana searched for the Druids, she found herself surrounded by Serkets. She was stung on the leg before being saved and cured by Aglain (The Nightmare Begins).
After Merlin was caught eavesdropping on Morgause and Morgana he was chained up by KingCenred's men. Morgause enchanted the chains so that the more Merlin struggled, the tighter they became. He was then left on his own surrounded by Serkets. He tried to break his chains using magic but failed. Then he just forced them away with magic, but eventually he was struck by a Serket sting in the back. Merlin called for Kilgharrah before falling unconscious. Kilgharrah then came to Merlin's rescue and killed the Serkets with his fiery breath (The Tears of Uther Pendragon).
In Egyptian mythology, Serket is the goddess of healing stings and bites who originally was the deification of the scorpion. In "Merlin", those creatures are scorpion-like and cause poisonous wounds.