PHB 276 said:Resist: Resistance means you take less damage from
a specific damage type. If you have resist 5 fire, then
any time you take fire damage, you reduce that damage
by 5. (An attack can’t do less than 0 damage to you.)
If the target of dark spiral aura has resist necrotic OR resist psychic, the resistance applies.
A handful of power that have mixed energy type requires the target to resist to all energy keyword from the attack for the resistance to apply, but in those case it is spelled clearly in the power. Most of the time, a single resistance is enough.
PHB Update said:Keywords [Revision]
Player’s Handbook, page 55
Replace the second and third sentences of the fourth paragraph with the following: “Also,
resistance doesn’t reduce damage unless the target has resistance to each type of damage from
the attack, and then only the weakest of the resistances applies. For example, a character who
has resist 10 lightning and resist 5 thunder who takes 15 lightning and thunder damage takes 10
damage because the resistance value to the combined damage types is limited by the lesser of
the two resistances.”