Sidereum

Sidereum comes from the earliest days of the game, added before the Main Update by ep1k_sol, and it is still a modest target: Epic tier, 1 in 4,096. That is roughly 4,100 rolls at 1x luck and about 410 at 10x, so a fresh account can reach it in a single sitting of rolling. No biome or event is attached, and the wiki lists it as obtainable. Its in-game text calls it a trail of broken stars.

Visually, an orange star sits in the chest while purple and yellow lines swirl outward from the player, and now and then a distinct purple crack animates in the floor. The wiki compares it to Natural but reversed: Natural pulls strings into the character, while Sidereum sends them out, and the purple and yellow colours also match the title font. The cutscene fades to black with an ominous sound, then the screen edges glow purple with yellow in the upper left corner. It has no active ability. A crafting table exists as well.