Short answer:
One that brings a hazard.
Explanation:
For reference, Sudden Call is a Minion resource card that states:
There is actually a digest by ICE that indirectly answers this question:You may play this card as a resource or a hazard according to The Audience of Sauron Rules. This card may not be played as a hazard against a Wizard player, and may be included as a hazard in a Wizard’s deck. You may reshuffle this card into your play deck at any time that it is in your hand (show opponent).
The resolution of this committee is that if a card may be included “as a hazard” (or other specified card type), and it counts as a hazard while being in the sideboard, then for all purposes, it is considered a hazard for that player, which means that an effect which brings hazards from the sideboard in order to retrieve it would be required, e.g. An Unexpected Outpost, or tapping a Nazgul permanent event. Furthermore, this means that a Hero or Fallen-wizard player can’t use Smoke Rings or tap their wizard to bring Sudden Call from the sideboard into their play deck because the card counts as a hazard for that player.ICE RULES DIGEST #11019-07-1998 NetRep: Craig Ichabod O'Brien SourceQUERY 9
From: Martin Toggweiler
Sudden Call text states it “may be included as a hazard in a Wizard’s deck.” Am I correct in believing that it also counts as a hazard when in a Wizard’s sideboard?
Yes.Does Sudden Call count as a hazard, resource, or either for purposes of FW (vs. minion) deck construction?
Hazard.
Does Sudden Call count as a hazard, resource, or either when in a FW’s sideboard?
Hazard.