http://en.uesp.net/wiki/Online:Irresistible_Damage
Irresistible Damage, also known as Daedric Damage or Oblivion Damage is a type of magical damage. True to its name, it cannot be resisted by any damage reduction effects. Fortunately, sources of Irresistible Damage are very rare, being found mostly as set bonuses. Note that Irresistible Damage is not subject to Critical Strikes, so it is less effective against lightly armored opponents but more effective against heavily armored enemies or enemies who are Blocking or using a Damage Shield.