Demon Charge is similar to a warrior's charge, which can only be used when the warlock has activated Metamorphosis. This is used primarily for the stun component, which does not share a diminishing return with any other warlock CC ability, other than the felguard's intercept.
Note that, for some reason (possibly a bug), this ability does not work on training dummies. Using it against them will stun them, however, you will remain stationary. If you are using the dummies to test your rotations, beware of this glitch.