Improved Sneak Attack
Type of Feat: (Epic) Class
Prerequisite: Sneak Attack 8d6
Required for: None
Specifics: Add +1d6 to your sneak attack damage. This feat may be taken multiple times, to a maximum of +10d6.
Although it's not specified in the manual or the in-game description, the prerequisite requires the character to have 8d6 Sneak Attack in any 1 class. A mixture of Rogue/Assassin with a total stacked Sneak Attack of 8d6 would not qualitfy.
As of patch 1.66, a bug prevents Improved Sneak Attack from working for an Assassin who does not have at least one Rogue level or four Blackguard levels. The Improved Sneak Attack feats are still selectable for the Assassin, but they will add nothing. If the Assassin has at least one Rogue level or four Blackguard levels, Improved Sneak Attack works as expected for him.