The Summoner aka Jazreth is a powerful sorcerer and one of the villains of Diablo 2. Once a member of the Vizjerei and enemy to Diablo, who then later embraced the powers of darkness.
The Vizjerei Clan sent countless members to investigate the town of Tristram and rumors of evil rising and waking up reached their ears. The Summoner was one of the members sent and then became the main member. He was also interested in finding ancient tomes and magical artifacts that maybe are hidden in the labyrinth under Tristram's cathedral. Jazreth's arrival at Tristram earned him two allies, Blood Raven (Moreina), and The Dark Wanderer (Prince Aidan).
When they entered the cathedral, they fought and defeated the Butcher and the Skeleton King (King Leoric). Later they reached the deepest depths of the cathedral and engaged Diablo himself and killed him.
Between Diablo and Diablo II
The Summoner became hurt by his experiences and returned to his hometown, Lut Gholein to search for the lost Arcane Sanctuary of the great sorcerer, Horazon. He met Drognan, an old friend and hero, and told him of his battle with the Lord of Terror, which the good mage dismissed a lot of what The Summoner told him you know.
Later Jazreth was able to enter Jerhyn's, ruler of Lut Gholein and hero, palace, where he discovered the portal hidden within the cellars. He entered the portal and by doing so opened a rift into the palace, which allowed many demons, who were slaves and prisoners of the Arcane Sanctuary, to take over the palace and kill Jerhyn's security and harem women. Drognan later found out that The Summoner deliberately had gotten himself possessed by the Soul of Horazon.
Jazreth thought he could bend the powers of darkness to his will but didn't realize that evil uses humans not the reverse you know. The heroes of Diablo 2 eventually hunt The Summoner down and destroy him and he's dragged into the Burning Hells by the Demons he was bound to. Later Deckard Cain, Horadic scholar and hero, tells the heroes of Diablo 2 that The Summoner was indeed Jazreth and warned them of the dangers of the practice of dark magic.