For the first step of the Exodus: Evacuation quest in Destiny 2 you will need to find Barrism, a powerful Taken Phalanx enemy. He is located in the Sanctum of Lost Bones and is a bit of a tough customer. This is part of the Observer Effect step for the exotic quest.
You can find the Sanctum of Bones directly across from Asher Mir on Io. All you need to do is turn around, run through the two strangely-angled pillars, and across the dirt road. The entrance to the Sanctum of Bones will be right there behind a large boulder with a Hunter symbol on it.
From there, venture into the Sanctum of Lost Bones and follow the only available path. You will have some enemies to fight, which is fine as it helps you collect the Traces of Corruption that you also need for this quest step.
You will come to a room that has a Taken Blight in it, and after that is a room with two platforms leading upwards with some Taken Psions on them. Take them out, then head for the tunnel at the top of the platforms. In here, you will find Barrism, and he has a friend with him, the Lost Sector boss. Take them both out in whatever fashion suit you, and you will get the Taken Sample when Barrism dies.
After than, make your way to the back of the room and open the Lost Sector cache to finish the final requirements for the Observer Effect stage.