Obsidian Sanctum in Guild Wars 2 is a long jumping puzzle in the middle of Eternal Battlegrounds (EB) map in GW2. This is a fairly long jumping puzzle and enemy players from other servers can kill you or grief you as you doing the puzzle. The end of the puzzle is a large chest that often give you 2-3 siege equipment blueprints and as well a nice sum of Badges of Honor (chest reset daily).
Getting to the puzzle
Your server must control a Keep in Eternal Battlegrounds for you to have access to this puzzle. You can enter it from all three keeps via a mystic portal.
While you are susceptible to enemy attacks when doing the puzzle, there are two fountains that grant you invisibility. One of them is just near the entrance (you need to drop down in the water and do a mini jump puzzle to get it). The second one is also fairly near the start of the puzzle (you have to climb a slope to get it). The rough locations are indicated in the picture.
First Mystic Fountain near the entrance (once you get on the roof of that structure, you will need to make a long jump onto the rocky ramp and climb up)
Second Mystic Fountain is up on a slope
There is also a shortcut when you are in the dark room, when you are on the beam near the end of the puzzle (before you do the double dodge-roll), if you have swiftness you can jump to this beam and avoid the entire double doge trap entirely.