Ancient Essence

Ancient Essence is currency that is used to upgrade items at the Cosmic Crucible.

Even in the early game you will randomly find small amounts of it: it sometimes drops from monsters, etc. Some artifacts can be exchanged for Ancient Essence at the shops of Science Outpost.

In the late game, there are two plentiful sources of Ancient Essence: Soulleaf Seed will allow you to cultivate it in greater quantities, while the Precursor computers (such as the Precursor Mainframe) will yield a small amount of Ancient Essence in addition to their normal Research yields. It's also possible to amass a large amount of Geodes (either by growing them on Geode trees or by buying them for pixels) and feed them to a Microscope, which will sometimes produce Ancient Essence.

You also get 2000 Ancient Essence by defeating an "Ancient Guardian" boss, which exists in several missions (at least Brine Star and Hydro Center) and in vanilla Ancient Vaults (see https://starbounder.org/Ancient_Vault ). These bosses are much stronger in FU, but their strength lies not in greater stats, but in their advanced tactics (which are randomized, so they won't be the same for each Ancient Guardian that you'll encounter). By adapting to those tactics you can defeat them.