Sicarai are known for their single-minded purpose and lack of mercy or forgiveness. They are almost undefeatable.
Many Sicarai were trained by Tyrael.
Sicarai were dispatched by Balzael after Diablo's failed assault on Heaven to eliminate pockets of demons wherever they could be found—the outskirts of Hell, the Pandemonium Fortress, and even Sanctuary itself (a world they had never graced up until now). They struck hard and fast, and dispatched humans that got in the way without hesitation. Balzael dispatched one Sicarai to eliminate Tyrael and his band of Horadrim, but was forced to withdraw. The Sicarai was eventually killed in a battle in Heaven.