Artifacts are more powerful than magical items. Unlike magic items, artifacts are neither offensive nor defensive. Some champions can be immune to artifacts. A single artifact can affect an individual champion, all champions in a pool, change the battle conditions, or even all of the above. In the Order of Activation, artifacts precede magical items because of their power.
Any special power of the artifact that would directly affect an opponent, such as its level bonus, is ignored by champions immune to it. When an artifact can affect only the attached champion or his attachments, another card's immunity to artifacts would not negate that effect.
Artifacts can be attached to the champions from the same world (logo) and during phase 3 only. You can not use them from the hand during phase 4. Champion entering combat from the hand does not have any artifact attached. Some rare artifacts negate this rule and can be used during combat. That is written on the card's special powers.