Encirclement

Pursuit and encirclement behavior

Mobs will attempt to avoid other mobs chasing the same target when not actively attacking. The result of this is that a horde of mobs will automatically fan out while still chasing down a target.
Mobs will attempt to stay out of your attack range unless they are preparing to attack. Combined with the above mechanic, mobs will begin to encircle the player to cut off escapes once close enough.

A small handful of mobs are permitted to attack at any moment, shuffled dynamically by a stage manager to prevent repeated attacks from the same target. This keeps up pressure on multiple fronts when encircled, while still allowing tactical play beyond kiting, unlike in a congregation-styled encounter where falling into the mob group generally spells swift death.
The attack power cap is currently hardcoded to 3. Each mob has an attack size of 1 when not using custom movesets, so out-of-the-box only 3 mobs will be allowed to attack at any given moment. Custom movesets may have their own size values that allow more or less mobs to attack simultaneously.
Mobs without custom movesets will be forcibly shuffled out of play after 5 seconds or after a successful attack to keep the cycle going.
When not attacking, mobs will hang around the outer edges, strafe around to potentially cut off escape routes, and in general look ominous.