Assassin (Lore)
The Assassins, also known as the Viz-Jaq'taar, that joined in the war against Diablo in Diablo II are not going to be included as a playable class in Diablo III. They are also known as the Order of Mage Slayers.
The Order of Mage Slayers was formed after the Vizjerei survived the tragedy brought on by Bartuc and Horazon. Their purpose was to watch for rogue magi that may become corrupted by the forces of Evil. To be effective at this task they were trained in abilities that were not conducive to Demonic corruption. Living as rumour and myth to the general population, even other magi knew very little about this mysterious order. Their reputation was shrouded in mystery and the simple fear of their retribution almost ensured steadfast vigilance against corruption amongst those who knew they were real.
Assassins do not employ the magical arts directly; rather, they use enchanted items that mimic elemental powers and can be used autonomously. To further avoid potential corruption, they focus on the natural abilities of the mortal body - powers of the mind and unarmed combat.
Diablo II Info
This is just part of the material available on this topic. The Diablo2Wiki Assassin (Lore) article is available in the Diablo 2 wiki, which has a specific Diablo I and Diablo II focus. A lot of information unique to the first two Diablo games is available there. You can find it here:
- Diablo2.DiabloWiki.net/(Lore) Assassin (Lore).
