Stellaris

You have to have the Enigmatic Fortress spawn in the galaxy, for my test runs it always seemed to be on the class A stars to help narrow the search. Once you beat it you have to start a special research project for it which has a lot of ways to fail it.

Stage 1 if you send a transport you reach stage 2 but the army dies. If you send a scientist go with the lower estimate. If you pick the higher estimate the fortress recharges.
Stage 2 you must rearrange the metal tori on the poles.
Stage 3 you can choose between The Middle, Dark Matter or Black Hole research project.

if any of these stages timer expires it repowers.