Surrounds the Death Knight in an Anti-Magic Shell, absorbing 75% of damage dealt by harmful spells (up to a maximum of 50% of the Death Knight's health) and preventing application of harmful magical effects. Damage absorbed generates Runic Power. Lasts 5 sec.
Also widely known by blood death knight community that timing AMS right before next harmful stack hits.. it dispels the existing ones.. been like this since.. forever ?
EDIT: sorry failed posting time :-) , but still WIDELY known and working as intended if you play Blood DK you should know this "trick"