Swarm

Gathers on whatever is hurt. It will bite a wounded unit alone, but it will not touch a healthy one until it has the numbers. It forces you to decide whether to pull a damaged unit back. A lone survivor cannot form an encirclement, so it stalls.

Activation

Always — active from the first turn.

Action Rules

It can attack at range and the wounded enemy is attackable from somewhere in Move Range
→ Take Max Standoff against the wounded enemy
The wounded enemy is within attack range
→ Attack the wounded enemy
It can attack at range and some enemy that stays encirclable without it is attackable from somewhere in Move Range
→ Take Max Standoff against the one with the most attrition
An encirclable enemy that no longer meets the stack condition is within attack range
→ Attack the one with the most attrition
It has a skill and a target meeting the stack condition is in range — for skills cast on enemies only encirclable ones count
→ Cast on the one with the most attrition
An encirclable enemy is within attack range
→ Attack the one with the most attrition
The wounded enemy within Sight has a measurable Move Distance
→ Advance fully toward the wounded enemy
The wounded enemy within Sight has a measurable Terrain Distance
→ Make a projected advance toward the wounded enemy
Some enemy has a measurable Move Distance
→ Advance fully toward the nearest by Move Distance
Some enemy has a measurable Terrain Distance
→ Make a projected advance toward the nearest by Terrain Distance
There is some attackable enemy on the board
→ Make a straight approach toward the nearest by Hex Distance

Rule 2 alone does not require an encirclement — a wounded unit gets bitten by a single attacker. On a turn when nobody is hurt the nearest enemy counts as wounded, which is where the first bite comes from. After that the same unit stays the most worn down and the swarm converges on it.

It does not take bases. Mixing in a unit that can capture will not send it toward one.