Astro

Astro is the crafted secret-tier slime, tied to the Crystal Cavern recipe, and at 1 in 2.47 billion it sits among the rarest builds in the game. As a Tier S body it is genuinely endgame, but the crafting path is the real story, because reaching Astro means hunting the recipe scroll and gathering ingredients rather than rolling for a clean drop the way you would for Bomber or UFO.

Pursue Astro if you are chasing the full crafted-slime lineage. After Crafty in the Cavern, Thorn in the Jungle, SlimeSlimeSlime in the Redwood Forest and Puffy in Candyland, the Crystal Cavern build is the next rung up, and the recipe pattern is finally muscle memory by the time you reach it. The honest comparison is to Bomber and UFO at similar tiers: those are pulls, Astro is a project.

By the time Astro lands, your account has effectively completed the mid-rarity crafted set and is staring down Sunny in Atlantis as the next build. Until Sunny is in your collection, Astro is the deepest crafted slime most accounts will ever finish, and that lineage is the reward.