There are three Circles of Slaughter that need to be completed. They can be found on Pandora, Promethea and Nekrotafeyo.
Pandora's is "The Slaughter Shaft" and the entrance can be found in Konrad's Hold.
Promethea's is the "Cistern of Slaughter" and the entrance can be found in Meridian Metroplex.
Nekrotafeyo's is "Slaughterstar 3000" and the entrance can be found in Desolation's Edge. This one is different than the rest, as you will need to go back to Sanctuary and discover a new planet.
Here's screenshots showing the locations of all 3 entrances.
https://imgur.com/gallery/gouYRgAIf you find you are struggling, you can go into your settings and put the difficulty down to easier to help you beat the circles of slaughter.
Edit: If you're going to leave a negative, let me know why. I get that the slaughter's are a buggy mess. However, if there is something that you feel is missing from the solution, let me know what it is.