Dyadus Chill on Bosses

Does Dyadus chill work on all targets, regardless of damage dealt by the attack? If so, what is the magnitude of the slow?

I don't see why it wouldn't. Chill effects that are not based on damage dealt (this includes things like Vortex's Chilled Ground DoT, Arctic Breath's regular Chilled Ground, and the like) are set to 10% slowing effect, as per the dev manifestos for the Chill/Shock changes in 3.0.0.