You can imbue your death attacks with magical energy.

Prerequisite(s): Int 13, Spellcraft 5 ranks, Arcane Strike, 1st-level Assassin, ability to cast arcane spells

Benefit: When you make a death attack which has been modified by the Arcane Strike feat, you may choose to convert the attack's sneak attack damage to energy damage of a specific type, such as fire or cold damage. If your target has a vulnerability against the chosen energy type, the save DC to resist the death attack increases by 2. You may use this ability a number of times per day equal to your Intelligence modifier. In order to convert the damage to energy damage of a given type, you must know or have written in your spellbook at least one spell which deals damage of that type, though you need not have the spell prepared.