Arcum Dagsson — ruling 1

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

Arcum Dagsson — ruling 2

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.

Arcum Dagsson — ruling 3

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.

Arcum Dagsson : règles, fonctionnement et interactions (MTG)

Arcum Dagsson is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: Target artifact creature's controller sacrifices it. That player may search their library for a noncreature artifact card, put it onto the battlefield, then shuffle.

How it works : Arcum Dagsson

Original English text — Wizards of the Coast / Scryfall

#1August 7, 2020· Wizards of the Coast
Arcum Dagsson's ability targets an artifact creature, not the player who controls it.
No other card is named in this ruling.
#2August 7, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#3August 7, 2020· Wizards of the Coast
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.
No other card is named in this ruling.