First Chaper Edit
As a monster appearing in Rolent's sewers, it's part of the game's combat tutorial section and it's the second enemy the player gets to fight.
Two variations of the creature exist with only a minimal difference in stats. The most prominent difference is that the first variation, only present in the second battle of the game, doesn't actually attack the player and merely flies in place until the characters kill it. The second variation, present in the last battle in that area, does attack.
Being part of the tutorial, the Moth Cluster isn't a particularly formidable foe, its only attack being an attack that does moderate damage from up close although unlike the Dirty Rat, physical attacks are ineffective against them and often miss. They exist mainly to explain the use of orbal arts to the player. Since the only way to reliably take them down is by hitting them with an orbal art, it's often considered a wise move during the last battle in the sewers to position Estelle and Joshua in the back row of the combat grid beforehand so they have time to hit the Moth Clusters with an art before the latter can get an attack in.