Assassin NPC

[DoodlesD]

What would be the best/quickest way to make an assassin NPC and what would its minimum challenge rating be?

[PersonMan]

An assassin can just be any sneaky monster or someone with a level or two of Rogue. An Assassin needs to be at least level 5 to get into the class, so CR 6 if you're using a base race.

EDIT: Assuming 3.5 standard DnD, that is.

[Snake-Aes]

System? Any specs on what constitutes an Assassin?

[Greenish]

Marrulurk, cr 4 monstrous humanoid from Sandstorm. 2d6 SA, Death Attack, good hide/move silently.

[DoodlesD]

Being able to use death attack. Basically taking 1 level in Assassin PrC. Challenge rating 6 might be doable as a BBEG for my party if they level up at least once before the end. Preferably they'll be 5 lvl 4 characters, so if there's something similar but more suitable that would be good.