Yokai Slayers

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  Title screen

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  Kitsune

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  Oni

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Inspiration

We wanted to turn the world’s biggest forum into a living game board. Yokai Slayers was inspired by the couch-co-op feeling of teaming up with friends, the daily ritual of MMO raid bosses, and the eerie charm of Japanese folklore. Reddit already has tribes, rituals, and inside jokes—so why make players download an app or create a new account just to play together? By building natively with Devvit, we transformed a subreddit into a dojo where anyone can jump in, learn a bit of mythology, and contribute real progress in minutes. The goal: zero friction, real community collaboration, and a respectful, stylish homage to yokai, complete with bilingual names, accurate lore, and an atmosphere that feels like stepping into a woodblock print that decided to fight back.

What it does

Yokai Slayers runs entirely inside a Reddit post via Devvit. When a user opens the post, the React front end loads in a webview and auto-authenticates with their Reddit account—no sign-ups, no installs. Each subreddit functions as its own “server”: at midnight UTC a daily yokai with a massive shared HP pool spawns. Players create a Samurai, Ninja, or Onmyoji and queue a 3-turn action plan (Attack/Defend/Special). The backend (Express + Redis) validates actions, computes damage with class modifiers, updates the community boss HP, and pushes real-time leaderboards and victory states back to the client. When the boss falls, everyone who battled gets a global XP bonus and the defeat is stamped for that subreddit’s history. Touch-first UI, instant feedback, and fully production-ready systems make it feel native to Reddit—because it is.

How we built it

Yokai Slayers was built with a spec-driven workflow powered by Kiro. A /.kiro/specs/ directory defines classes, bosses, and the damage formula; Kiro then generates typed constants, shared types, and the combat engine, keeping client and server perfectly in sync. Balancing is a one-place change: edit the spec (e.g., Ninja bonus-turn chance 30%→25% or Kappa HP 100k→150k), run npm run kiro:gen, and TypeScript highlights any remaining touchpoints. We also ship pre-commit sync checks to prevent spec/code drift and a traceability doc mapping spec lines to generated files. Result: 81% less time on setup, iteration, and bug-fixing; more time on polish and playtesting. The repo includes repro scripts and a reusable /.kiro blueprint so other Reddit-native games can adopt this approach immediately. In short, Kiro turned balancing a community RPG into editing intent, not wrestling code—fast, reliable, and repeatable.

Challenges we ran into

I wanted to integrate AI-generated Yokai, so that there would be limitless possibility for boss battles, but I was unable to get my API url approved in time for my app. I built this but I could not test it. See here for how the API works - it creates a new image, description, and stats for a Yokai each day: https://github.com/contrabandinteractive/yokaislayers/blob/main/api/generate-yokai.js

Accomplishments that we're proud of

I think this is a great starting point for a community-driven game that can be addictive, fun, and rewarding. There is an interesting turn-based battle system that I think will serve as a great foundation for this game.

What we learned

Kiro really opened up my mind to new possibilities when it comes to agentic coding. It's really made me evaluate and think about how I can be more productive in my future projects using Kiro. Plus, it was interesting to build a game on Reddit - this is also something I never thought would be possible.

What's next for Yokai Slayers

I want to expand upon this and have overworld maps, more community functionality, and incorporate the AI feature I listed above in order to add unlimited Yokai battles.

Built With

• devvit
• kiro
• reddit
• typescript