The Hall of Unity popped up in the game today, after playing a while. No planets conquered, so that's not a prerequisite. But it didn't appear on all my core worlds, so there seems to be a hidden / undocumented prerequisite. Or a bug where some buildings in the available build lidt appear and disappear. I wanted to build it on a specific planet, and destroyed another building to make room. That made it disappear from the build list. I scrolled around a bit between planets and other stuff and popped up the planet again, and now it was back.
Ah well, I guess bugs are to be expected.