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Jack Leavey
Technical Designer
The AI of HOST
Unreal Behavior Trees
HOST features 8 unique enemies (including a final boss) all brought to life using Unreal Engine's Behavior Tree system. This deep dive will walk through some of the highlights of the game, and how the AI makes decisions using the Environment Query System (EQS) and player actions. Specifically, I will be breaking down the Obscura and the Thing, our final boss.
Obscura - Hybrid Flier
The Obscura is our resident flying unit in HOST - capable of both ground and air based movement and attack patterns. Here is the overview of the tree itself - we will dive into each section in more detail next. We use a series of selectors, sequencers, decorators, tasks, and services to bring this winged nightmare to life.
The default behavior is quite simple - the Obscura will either be using predetermined path points (if defined), or will define their own patrol patterns by getting random locations as needed.
Following this is the "Investigation State" - triggered by player actions. The Obscura can see, hear, and detect the player through proximity innately. If the player deals damage to the Obscura, the Obscura is also fed information about the attacker. The Obscura will also also check the area around where it detected the player (assuming it does not fully engage the player) - giving it a light "searching" state.
Finally, we move into the Engaging state where the Obscura determines how to do combat with the player. This includes determining if/when to fly, and what attack to use. The Obscura's general plan is to go airborne and fire its disorienting ranged attack, blinding the player. It will then land and go in for a devastating melee attack.
The final 3 states are quite simple and I will address them as such. The Crowd Control state handles a brief stun to the enemy, the Consumed state is used for when an enemy is able to be consumed, and the Dead state handles death logic. These are fairly uniform across the enemies.
The Thing - Final Boss
The Thing is quite a bit different than our other enemies in several ways, though chief among them is its lack of movement. This fight is broken down into several phases. Each phase triggers some new behaviors, attacks, and weaknesses for the boss.
Phase 1 begins by default, with the boss selecting between 2 possible attacks. There is also a branch to trigger the boss' weak point.
Once a set amount of damage has been dealt, we move onto stage 2 and the fight begins in earnest. The boss gains access to several new attacks to cycle through. It also retains the branch to expose its weak points. Attacks are given cooldowns and specific triggers that are callable from Boss' Blueprint and the Level Blueprint as well.
Stage 3 is more of the same - with the final few attacks enabled.
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