Destiny 2 has lost hidden sectors everywhere. These locations are full of enemies and if you kill them all and take out the Major at the end, you'll have access to a cache with loot in it. They can also combine with all kinds of activities, like Spider bounties, exotic quests, and other missions that you will come across throughout the game.
Destiny 2 - Where to find the Bone Sanctuary
Fortunately, the Sanctum of Bones is pretty easy to find. You'll need to get to Io first and spawn at the point near Asher Mir. Once you get there, turn around and walk between the rocks that are flush towards the opposite cliff.
You should be able to see a glowing light in front of you, next to the Lost Sector symbol. Make your way behind this group of stones and you will find an entrance in the wall.
Enter the tunnel and enter the entrance to the Sanctum of Bones. If you're here to hunt Pandrol, the Pillar of Nothingness, you'll need to venture out the back. Go up the ramp into the first cave and follow it all around, through the tunnels and past the Blight area. Jump to the other side of the stone platform to reach the tunnel and continue to where Pandrol likes to hang out. There are a lot of enemies in this Lost Sector, so if you're new to the game, you better take it slow. Simply take out enemies as you go through caves and tunnels.
And that's it, like I said, not the hardest lost sector to find, as long as you know where to look. Good luck, Guardian.