Yesterday my first time on Lam i put together a Tiefling Assassin VKF build and tested it at level 15.
Lasting Darkness seems to work as it says except for the chance part. Everything that entered was blinded and set up for full out sneak attack damage.
The animation looks to be the same as glitterdust very slow especially w/o quicken.
With a measly 100 SP trinket i was able to use it ten times but rarely needed to.
I am going to run further tests in higher reaper modes to see how far it goes. I have to say it is a lot of fun so far