Arcum Dagsson

Arcum Dagsson's ability targets an artifact creature, not the player who controls it.

Any triggered abilities that trigger when the creature is sacrificed will be put onto the stack at the same time as any triggered abilities that trigger when the noncreature artifact enters the battlefield.

If the target artifact creature is an illegal target by the time Arcum's ability tries to resolve, the ability won't resolve. No player searches their library. If the target is legal but not sacrificed (most likely because of an effect such as that of Sigarda, Host of Herons), its controller does search their library for a noncreature artifact card.