While I haven't done formal testing on proc rates for this weapon, the Endrain is a 200-300 HP drain effect, and I would say the proc rate is around 5-10%, based on my use of it.
I enjoy my Cronus, If you unlock the Quietus Weapon Skill it is teh awesome! Have fun with your Apathy and downward spiral into insanity and oblivion. (had to say something depressing)