An Implant Without a Source

Implant: Eternal Engine exists in the game's item data as a permanent key item, but no shop, loot table, or reward currently hands it out — and the Pal Surgery Table never asks for it. The passive is wired to the disposable version instead, so treat this item as an unused leftover: you can't get it, and surgery doesn't need it.

How to Get Eternal Engine Instead

Eternal Engine (Max Stamina +75% — the top of the stacking stamina family above Fit as a Fiddle and Infinite Stamina ) is applied with Disposable Implant: Eternal Engine : the surgery itself is free, but the implant is consumed. Disposable implants come from the Ancient Relic Recycler — every recycled Ancient Relic of any grade rolls one implant from a 16-implant pool, so Eternal Engine lands at 6.25% per recycle. Because it's one-and-done, spend it on the travel mount you've committed to. See the Ancient Relics guide for the reward tables and the implants guide for how surgery works.