Formation movement
Makes a group of units move together in a set pattern — line, wedge, box — while still avoiding obstacles and each other.
Each unit is given a target slot relative to a shared anchor (the formation's centre) and steers toward its slot with ordinary steering behaviours while the anchor follows the overall path. The hard part is holding the shape when terrain forces the ranks apart: some solutions scale the formation, others let units temporarily leave their slot and reclaim it.
Real-time strategy games like Total War build their whole feel on believable formations, and military simulations use them for fire support. It is related to flocking (boids) but imposes a structure instead of an emergent swarm.