Hierarchical task network (HTN)
A game-AI planning method that breaks high-level goals into subtasks until only directly executable actions remain.
The idea comes from classical AI planning in the 1970s (NOAH, NONLIN) and was formalised in the SHOP and SHOP2 planners by Dana Nau and colleagues. An HTN planner starts from a compound task — say "defend the point" — and picks among hand-authored methods that split it into smaller tasks, recursively, until every branch bottoms out in primitive actions the agent can actually perform.
Guerrilla Games popularised the technique in games with Killzone 2 (2009) and later Horizon Zero Dawn. Compared with GOAP, which searches the state space freely, HTN is faster and more predictable because the designer wrote the hierarchy by hand — the price is less emergent behaviour.