The Assassins, also known as the Viz-Jaq'taar (and sometimes Khral-Harzhek) - the Order of Mage Slayers - joined in the war against Diablo in Diablo II. They will not be included as a playable class in Diablo III.
The Order of Mage Slayers was formed after the Vizjerei survived the tragedy brought on by Bartuc and Horazon during the Mage Clan Wars. 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.
The order has been watching over the powerful mage Ormus for quite some time, thinking his madness could possibly be a demonic corruption taking over. It's possible they keep tabs on many individuals in this manner.
- Natalya - Placed in Kurast Docks 20 years ago in the demonic invasion. Was forced to stay in the docks awaiting orders while the heroes saved the docks and killed Mephisto. Presumably met up with the heroes again in Harrogath, or she might have defended some other Barbarian outpost, seeing as the Assassin hero is sent there.
Name of the Order
While Viz-Jaq'taar is the most commonly known name of the order, Deckard Cain has been known to refer to them as Khral-Harzhek as well. If this is a competing order, the name of the order in another language, or how these relate, is unknown.
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: