Rupture is a damage-over-time Assassin ability. Its effect is similar to the Bleed caused by Lacerate, but more powerful: the Ruptured effect deals a target 20% Melee DPS every second for 10 seconds (for a total of 200% Melee DPS in that interval.)
Can be learned from a Scroll of Rupture. You can buy one from a Combat Trainer for 50 silver.
Talents that amplify Rupture: