“A giant vulture swoops in from the sky toward its prey, only to be ensnared by kelp and dragged into a watery abyss to be devoured by leeches. A scavenger grabs an insecticidal spore puff and rushes into a swarm of flesh-eating locusts to rescue its tribemate. A lizard dunks its prey into a vat of acid to make sure it's dead before lugging it back to its den.” – Andrew Marrero, Development Lead, Akupara Games
Rain World’s grueling survival-based gameplay thrives on unexpected scenarios. While every one of these situations is possible in-game, none of the interactions are explicitly programmed actions or events – they’re the natural result of carefully crafted dynamic behavioral systems and creature-to-creature relationship systems.
We interviewed developers from Videocult, Akupara Games, and Rain World’s modding community to learn more about the creatures and environments in the game and its latest DLC The Watcher, and the procedural design and systems that bring them to life.
Rain World’s core gameplay loop involves navigating a complex and dangerous ecosystem as a lone slugcat separated from its family. Players must hunt weaker creatures for food, avoid and outsmart predators, and seek shelter from regular downpours of acid rain.
“From the start, we wanted scrappy, improvisational gameplay focused on stealth and occasional bursts of tooth-and-nail survival,” says Joar, creature consultant at Videocult. “The game is very much a knowledge-based one, using trial and error to learn the mechanisms of the creatures and ecosystem, and gaining a feel for how to manipulate it all in your favor.”
Learning how to effectively control the slugcat is a big part of this journey. In-game, the slugcat moves with a slippery, liquid quality, and can bounce off walls, slide across the ground, and squeeze through narrow tunnels. Although difficult to fully grasp at first, the slugcat’s unique movement system gives Rain World’s action sequences a feeling of “desperate scramble” and enables “some interesting interactions that wouldn’t be possible if the character was just an axis-aligned box.”
Visually, this is all enabled by procedural animation. The slugcat’s character model consists of two spherical chunks locked to each other at a fixed distance, enabling it to tumble and rotate. The limbs and tail are cosmetic additions drawn on top of this base, procedurally animating based on the player’s inputs.
“I never really set out to do procedural animation,” says Joar. “I was posed with the problem of making the slugcat’s limbs move, and since code is how you make things move in a computer game, that’s what I did. Early on I used a mixture of classical and procedural animation, but later I gravitated more towards procedural.”
The slugcat's animations in action
Rain World’s bestiary includes over 100 creatures with significantly varied appearances, behaviors, and habitats. While they share a few base creature scripts in common, for the most part, each is approached like a brand new project and coded from scratch. This adds extra development work, but allows for the biodiversity that makes the game so compelling.
“Some creatures have little stubby legs, while others have lanky nightmare spider legs, fins, jet propulsion, tentacles, and wings,” says Benjamin, creature consultant at Videocult. “Because the creature designs vary so much from each other, they require unique code to form their bodies and handle their unique ways of movement.”
This eclectic design approach is reflected in an unconventional Editor setup. ”Rain World is very strange in the way that it uses Unity’s Editor,” says Benjamin. “There are basically just input/output handling MonoBehaviour scripts on a few empty GameObjects, the main few being the screen panel, the sound pool manager, and the controller manager. Our creatures are created almost entirely from scripts, except for sprites, which are positioned and colored through the creature’s graphics script. Each creature inherits from base creature scripts, which handle collision and other general-purpose tasks they all need, such as health systems, held items, and saving/loading them.”
To establish creature-specific behaviors, each is given a table cataloging its relationships to other creatures, objects, and locations, with this information forming the basis of Rain World’s ecosystem. “On a behavioral level, creatures use a modular collection of AI modules that can be optionally attached and configured for each creature to change their possible behaviors and priorities,” explains Andrew, development lead at Akupara.
Some of these modules include:
• PreyTracker to hunt nearby prey
• ThreatTracker to flee from nearby predators
• FriendTracker to form cooperative squads with other like-minded creatures
• ItemTracker to find usable items on the ground to pick up and carry
• DenFinder to give the creature a sense of a home base it can return to after certain tasks
• DiscomfortTracker to define what areas or objects the creature typically avoids
The believability of Rain World’s ecosystem relies heavily on the emergent behaviors these systems enable, with some surprising even the developers: “I was making a poisonous, caterpillar-like creature, and another dev on the project had the great suggestion of making the poison be a way to remove parasites,” says Benjamin. “It ended up making its poison able to work as a kind of medicine – I thought that was really cool!”
The caterpillar's venom can be used as an antiparasitic medication
Maintaining this intricate system requires a balance between purely systems-driven interactions and occasional manual interventions from the developers. ”As the programmer behind these behaviors, I never saw anything I couldn’t explain, but I was frequently surprised by different ways things would lock into each other,” says Joar. He recalls one instance when a large tentacled enemy, the Daddy Long Legs – meant to be a major threat – became fixated on hunting tiny worms and completely ignoring the slugcat, a problem solved with some minor code adjustments.
While the goal was to create as few of these handcrafted interactions as possible, Joar acknowledges that some level of direct input ultimately makes Rain World’s environments more realistic: “For the most part, we tried to let the system play out as it’d play out, but we eventually gave in and created a tool where certain rooms could be assigned as more attractive to certain creatures, to give a bit of cohesion to the world.”
A vulture steals the slugcat's lunch
Rain World’s latest DLC, The Watcher, introduces 25 additional creatures, new biomes, and another playable slugcat equipped with unique abilities. “Our new Watcher slugcat has the ability to temporarily hide its visual form completely from most other creatures,” says Benjamin – of course, this doesn’t guarantee its survival. “There are some creatures with ways of getting around that however, such as hunting by sound, or painting the slugcat with their own naturally made pigments.”
The Watcher slugcat also comes with a new “Ripple” ability, a stacking buff that unlocks new powers as players progress. Ongomato, technical artist at Akupara Games, describes how they created VFX for the “Ripple Effect” by distorting, recoloring, or even replacing parts of the environment using an intensity mask:
“This mask is rendered first, captured, and then the frame is cleared. A Command Buffer then distorts or replaces the level texture based on this mask. The actual game scene renders using this altered texture, ensuring other effects that sample the level texture draw correctly and characters remain unaffected for visibility,” Ongomato explains. “The player’s distortion trail uses the same effect: Their silhouette is added to a texture each frame, slowly distorting and lowering in intensity, creating an organic trail that acts as a distortion source for the ripple effect.”
The “Ripple Effect” rendered in-game
Rain World’s design philosophy, rooted in procedural systems and emergent interactions, ensures that each run offers a fresh experience, reinforcing its appeal and replay value. This has nurtured a dedicated player community, who breathed new life into the game when Videocult opened the project up to modders. The resulting player-driven content generated even more interest in Rain World, with the most popular mods being integrated into the project as official expansions: Downpour in 2022, and today, The Watcher.
Closing out the interview, we asked Rain World’s dev team how they kept this game’s wildly ambitious scope in check over the years. Their reply? “We’ve never. During development we came up with ten thousand cool scope-creeps, and then put them all in the game. Note: not advice.”
Rain World: The Watcher launches today on desktop and console platforms. Check it out on Steam, and browse more Made with Unity games on our official Steam Curator page. Read more stories from devs in the Unity community on our Resources page.
