Does Sudden Storm deal 4 damage when a unit attacks an open battlefield?

Question

Accepted Answer

No. If a unit moves into an empty battlefield and only causes a Showdown (no Combat), it is not considered "attacking" for Sudden Storm and the spell deals 2. Sudden Storm deals 4 only when the target is an Attacking unit in a Combat (i.e., part of an attack against an opposing unit).

If the unit is on a battlefield that does not have opposing units (a Showdown without a Combat), Sudden Storm deals 2. It deals 4 only when the unit is an Attacking unit in a Combat (for example, in a Combat Showdown where there are both Attacking and Defending units).

A unit moves to an empty enemy battlefield. The move applies Contested and can cause a Showdown to be staged even if no opposing units are present [446] [323.8] [FAQ:unleashed#6]. Because the battlefield has no opposing units, this is a Showdown without a Combat (no Attacking vs Defending groups exist) [446]. You may play Sudden Storm in a Showdown (it has [Action] "Play ... in showdowns") and, as a Hidden spell, it must be played to that battlefield and choose targets from among options at that battlefield [811]. Sudden Storm's text reads "Deal 2 to a unit at a battlefield. If it's attacking, deal 4 to it instead." The conditional "If it's attacking" requires the unit to be an Attacking unit in a Combat context; a lone unit in a non-Combat Showdown is not an Attacking unit, so the spell deals 2.

If the Showdown becomes a Combat Showdown (for example, opposing units arrive or the Showdown is initiated as a Combat Showdown per choices during cleanup), then the unit can be an Attacking unit and Sudden Storm would deal 4. Also, if other rules or card effects in the supplied environment explicitly define a unit as "attacking" outside of a Combat context (not present in the provided core rules), that could change the result — but no such definition exists in the provided text.

The important point is what "attacking" means in the rules you provided. The Core Rules show that a unit can cause a Showdown even when there are no opposing units present (a Showdown without a Combat) [446]. The cleanup rules mark such Showdowns as staged when Contested is applied [323.8], and the official clarification confirms Showdowns can be opened when there aren’t units controlled by different players there [FAQ:unleashed#6]. In that situation there are not both Attacking and Defending units at the battlefield, so there is no Attacking role to apply [460.2].

Sudden Storm is playable in showdowns (it explicitly allows play in showdowns) and is Hidden, so when played from Hidden its targets must be chosen from options at that battlefield [811]. Its text distinguishes between "deal 2 to a unit at a battlefield" and "If it's attacking, deal 4 to it instead." Because a lone unit that moved to an open battlefield and caused a Showdown is not an Attacking unit in a Combat, the conditional is not satisfied and the spell deals 2 damage. If the battlefield instead had opposing units such that the unit was an Attacking unit in a Combat (for example the Showdown is a Combat Showdown with Attackers and Defenders), then the unit would meet the "attacking" condition and Sudden Storm would deal 4 damage [460.2].

Therefore: playing Sudden Storm targeting a unit that simply moved into an open battlefield (causing a Showdown without Combat) deals 2 damage. It deals 4 only when the target is actually an Attacking unit in a Combat.