It is well known fact by now that the rouge assassinate ability is largely useless at epic levels, and rarely works if at all in epic quests. This is largely due to the fact that monsters have increasingly high fortitude saves while assassins have very little in the way of enhancements, or items to use so that their assassinate DC can keep up. I think there may be only two items in the entire game that increase assassinate DC. Assassinating is really the whole idea behind the prestige class and is a large part of what makes it fun to play. Please get this fixed as soon as it is feasible.
Several things could be done to fix this and I would greatly appreciate any or all of them. The base DC of the assassinate skill could be increased by a set amount once the rogue hits level 20. This base adjustment would allow the skill to continue being useful and wouldn't cost the rogue extra action points or item slots that they're already using elsewhere.
Assassinate DC could be made an enhancement found on random loot much like seeker, backstab, etc. It would have to be enough of an increase to make a significant difference though.
More enhancements in the base assassin tree in epic destinies, or both could make enough of a difference to make the skill useful again. This is also an option but as I said before would take up AP or destiny points they may already be using other places. If increases in assassinate DC were added to existing enhancements, that would fix the problem without having to use the extra points. I hope you see and consider this soon.
Thanks for all the work you do on this great game.