The Achievement Hunter - FTL : Multiverse 5.4 - Part 1 

It adds more renegades to the pool.

To add to this, its not just a higher chance of renegades it adds beacon unique renegades. You can see which ones are unique to the beacon on the achievement screen.

@@Witherbottom Oh that's what I meant: I didn't realize it's also a higher chance, all I know is that it just added more variety.