The Quick Kill (2008)

From the introduction:
The first release in the Dark Emerald line of 4th Edition products for the world’s most popular roleplaying game, Dungeons & Dragons™. These pages contain an optional rule for killing opponents in their sleep and includes three encounters for use in any heroic- or paragon-level adventure as a bonus scene set within an existing adventure. Your players can practise that killing blow for themselves or avoid falling victim to it, all of which are provided here as examples for how this rule can work.
Known as the quick kill rule, it allows a stealthy character to sneak up on a sleeping or unsuspecting opponent and kill them instantly. Using their knowledge of humanoid (or even aberrant) anatomy, a well-trained killer can use this optional rule effectively to gain the upper hand in the adventure. Doing so requires skill, patience, and talent to pull off and has its own rewards, as well as its downfalls. While it is not designed to replace the coup de grace attack as written in the Player’s Handbook, it provides a viable alternative.
Following this material, three appendices are provided with fully detailed encounters applying this optional rule with the PCs as the potential victims or the willing killer. These encounters can be inserted into any campaign and modified accordingly and are provided simply as examples for groups
to try out and determine for themselves if this rule is a worthy addition to their table.