Megalesios is one of the three Telkines. He is a powerful sorceror with several lightning attacks. You encounter him in The Room of the Conduit, at the end of Act 1. He is also the Telkine that appears in the intro movie of the game.
At the beginning of the battle, he erects a shield around himself that you cannot pass through, and he animates four guardian statues to protect him while he destroys the Celestial Conduit.
Megalesios' attacks depend on how close the player is. If you are far away, he has two attacks. The first is a slow moving projectile that disrupts skills for a short time. The second is a variation of the thunderball that splits into small spots of electricity if it hits. If the player is close by he will attempt to use a blast that is extremely accurate and does a fair amount of damage, but has no special effects. He also has an electric energy wave that he will only use if you are right next to him.
Every once in a while he casts a spell to take control of any nearby player pets (like Wolves). The affected pets will have a small red skull icons above their heads. However, this effect is temporary and lasts only a few seconds. Occasionally he also summons two Limos that leech your life and feed it to Megalesios.
You should have no problem with the Guardian Statues, as they are very weak. As soon as Megalesios summons the Limos, kill them instantly to prevent them filling up the Telkine's life. Also improve Vitality resistance to prevent them from leeching too much life. With 60-70% lightning resistances, Megalesios is very easy to defeat.
The mind-control of any pets is temporary and lasts only a few seconds. Also, if you move a certain distance away from Megalesios, the pets break off the chase and return to their temporary master. When the effect wears off, the pets are teleported back to the player's position. You can also use his own attack against him by using the Spirit mastery and mind controlling one of his Limos.