One of eleven Isle of Siptah achievements, this used to be completed with Wild Surges after a Maelstrom event. After Wild Surges were removed, this achievement was relocated to a very obscure place. This quick guide will help you get the elusive achievement.
How to Obtain The Outsider Achievement
This achievement has been frustrating a lot of people, and after some digging, experimenting and eventually succeeding, I thought I would share how I managed to complete it.
You're going to want to head to the Pools of the Grey Ones in the South East portion of the Isle of Siptah.
At the southern end of the complex you will find a large pool with a pedestal. This pedestal is where you use the figurines you may have been finding around the island.
Specifically, we need the Spider of Leng Figurine. These figurines appear to spawn in static locations, but the figurine that spawns is seemingly random. If you haven't found one yet, you can always hit up the admin panel and give yourself one.
Interact with the pedestal and drag the figurine into the window. This will summon the spider boss.
When Leng spawns, you must wait until he spawns adds. Do not kill the boss. Just dodge roll, run, and generally evade combat. It can be a bit before adds start to spawn, but they definitely will.
Once the adds spawn, kill one of them for the achievement. Afterward, deal with Leng or run for your life. If you are doing this achievement the old fashioned way, be aware that Leng hits like a freight train.
Enjoy your frustrating and not at all logical achievement!