You can equip the assassin with poison, which increases its CR by +1. On the assassin's first successful attack, the target must succeed on a Constitution check, DC [10 + CR], or take [CR x 1d6] damage and become Weakened for the rest of the combat. On a success, it takes only half damage and is not Weakened.

An attack similar to the Rogue's Sneak Attack that must be made from behind or by surprise. The creature gains a +5 bonus to its attack roll and adds [2d6 per rank] to its damage. The creature also gains a +5 bonus to Stealth checks.

The creature becomes invisible and can move 20 meters. It reappears on its next turn, and if it wins an opposed Stealth (Dexterity) check against its target's Wisdom, it can make a Deadly Strike.

On the first round of combat, if the target is Surprised, a successful Deadly Strike forces it to succeed on a DC 15 Constitution check or drop to 0 hit points.