Best Dagger Ever Update:
After saving AP (hiding and healing) for a couple of turns, I was able to Phoenix Dive into the next room of the cult dungeon and kill three (wounded) enemies in one turn, even though I had to use AP to move between targets.
Sweet story about your uber dagger find. That is just crazy amounts of damage!
The scary part is that the dagger was two levels below my character and how little Single-handed it required.
I don't think that the Assassin skill you mentioned is entirely a bad thing for the dagger weapon in general. You still have to figure there is a much higher AP cost for using "any other weapon" while trying to backstab. Though, with your technique of teleport and then backstabbing, it is possible that might be entirely too OP with the higher damage on all other weapons that would have backstab utility.
What would you suggest for the skill then, to have it removed or altered? Like 1 hand weapons only?
I think removing the skill might be the answer, but I should try making a 2H rogue with Assassin first.
edit: If there isn't already (didn't dig to hard) but there could possibly be a modifier for mobs or certain enemies that are unable to be backstabbed, or have really high resistance to it or something, maybe even an Auto-Counter to a backstab attempt (hightened senses etc). That could counteract the OP'ness of a "Backstab with any weapon skill" character maybe? Just a thought.
That wouldn't counteract "Backstab with any weapon skill", it would counteract backstabs with a dagger as well.