The Universal Union (UU) is an interdimensional alien empire. If you're unfamiliar with HL2RP and don't want to spoil it, I'd suggest you to not read this
The UU is simply the inter-galactical space unit, or otherwise known as Advisors.
In Half-life 2 RP, this focuses on Earth, where they cannot live, so they're in Stasis pods, on life support.
The Advisors have got a tongue, they can use to plant it into your brain and collect knowledge you have.
For as far as anyone is aware, who have played HL2RP before, including me, we've never seen ANY Advisor in an HL2RP themed server, mainly because the Model has got no Animations whatsoever, we hope that kurozael is going to fix this in the future.
If you've played Half-life 2, You know about Breen, and Breen has been named as Earth Administrator, but the fact is, he's got power over the earth, but the Advisors got the power over hím, and that's also seen in the end of Half-life 2, where he makes a plea for a safer destination of the portal, which is denied by them. There are many references to the Universal Union, they're mainly found on the Pods of the Advisors, which will say: 314 URB-LOC 0017.
314 - A possible identification of Earth to the Universal Union,
URB, which is for URBAN
LOC, which is for LOCATION
and 0017, which is for City 17.
For the rest, there's little known about the UU, all we know is they are the ones that invaded Earth and took it over.
Some see them as our savior, some see them as our oppressors...
It's all up to you...