As the title states. In my current game (latest 2.8 update), the event placed them next to one of my planets that was formerly another civ's capital. The next turn I checked back at that location to see what their ships were doing (they turtled at their planet), and I noticed the tooltip showing that my culture was affecting their planet (55.1x, 0% rebellion, normal in that regard for a newly colonized planet in an established territory). And the turn after that their planet was mine with no effort expended. Pretty sure this was not intended, not sure if it had already been addressed with all the sweeping changes coming in 3.0.
I do have some mods on, the PopBalance mod and the MercRoleMod, but neither of these should affect the Dread Lords as far as I know.