Summoners are a uncommon enemy type encountered in Prince of Persia: The Forgotten Sands.
During the Prince's journey through the sewer systems, a Summoner appears and conjures an army of Wraiths to attack the Prince. The Prince was only able to stop the warm of enemies attacking him by killing the Summoner.
Summoners have the ability to conjure Wraiths and Specters from the ground. It is a good idea to kill them first. While normal sword attacks will do, it is more effective to kick them down and stab them. Be sure to move quickly, or else they'll perform an Electrical Shock attack and push you back.
Another alternative you can do is simply take on the hordes of reinforcements and gain extra XP. Though Summoners are not limitless in creating reinforcements.