A rogue-type character with a "ki" version of Hide in Plain Sight available at every level.
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With any sneak-attack build you seriously have to question overall value. After-all, a Warlock essentially has sneak attack damage that is unlikely to be resisted. A Warlock can also have unlimited stealth-like capability from Invisibility. One level of Rogue and the feat Able Learner can even give you most of your Rogue skills. So any Rogue-type character like we have here is more about role-playing value rather than pure combat capability. Still, a Sneak Attack has the advantage of multiple attacks per round - and is even available early-on with Rapid Shot on (which requires Rapid Reload for the added attack). So two attacks per round at level 3 with 2d6 of Sneak Attack pretty much means "death" for any opponent at that level. (..a 3rd attack at level 10, a fourth at level 17, etc..) Warlocks can use Eldritch Glaive (for a max of 3 melee touch attacks), but those added attacks are latter and again, "cap" at 3 attacks per round. (..plus: Eldritch Glaive takes a full round to "cast" for each round of attack.)