Rogue Galaxy Quarry: The Envoy From on High
The closest save teleporter to teleport to in order to reach The Envoy From on High is Gulza Sanctuary - Plaza. The summon circle with the ? is literally right here - there's no extra navigation needed! If you're having trouble finding it somehow I still took a screenshot of my location, the page would just look dorky without it!
The item required to summon this boss is a Sealing Sunsphere.
If you are missing the item required to summon this boss grind some battles and cross your fingers or go treasure chest hunting on a planet you've 100% the map on. In Rogue Galaxy once you pass a certain chapter the loot table of items that can drop for you ingame is updated.
What I mean is, every item has a chance of dropping off every creature after you've passed that chapter that allows that item to drop. At least this is how it works in my experience. Therefore, if you're missing any items to summon any of the quarry bosses all you need to do is grind a little until you get that item to drop for you. Good luck!