1. lasting for a very short time.
Not only is this an undead swarm, it's an incorporeal undead swarm! This rare combination of types and subtypes is commonly referred to as 'all the immunities'.
The ability scores are a mixed bag. Intelligence is ridiculously low, wisdom is average, and strength and constitution literally nonexistent, but dexterity and charisma both get a decent +8 bonus.
In violation of the actual swarm rules, the ephemeral swarm deals 1d6 strength damage to creatures caught within it. The typical Distraction ability works as usual.
However, considering the 12 undead RHD that come with playing a creature like this, I can't in good faith assign more than -0 here. Being an incorporeal undead swarm may be nifty, but at this cost it's not worth it.