Age of Wonders: Planetfall

Essence Charges are introduced in the Revelations expansion, and they essentially function as mana, as they can be used to power some unit and mod abilities. There are a number of units that can acquire Essence Charges, all of the Heritor units for example, as well as a number of unit mods that allow the modded units to also acquire Essence Charges.

Essence Charges only track for this achievement when they are acquired through manual combat unfortunately, which makes this achievement a bit of a slog. There is an easier way though. Play as a commander with Heritor tech, and launching the Heritor doomsday weapon (both of which you can do during the second Revelation campaign mission). One of the effects of the doomsday weapon is that all of your units will gain their maximum number of Essence Charges at the outset of (manual) combat. So build Heritor units (collecting Drained via the tactical ability is recommended), and mod the standard units through-out the mission. Once the super weapon is active move your army(s) into position near any enemy occupied hex. Quick save the game. Then start manual combat. An army composed of 6 Drained will earn you 24 Essence Charges, just under 5%. You can confirm progress through achievement tracker. Re-load and repeat as needed. Then more units and armies the faster it will go.

I managed to get to 53% during the campaign before launching the doomsday weapon, then attacked an enemy colony with 3 armies and earned 12%. Then I rinsed and repeated til it unlocked.