This was a tip I stumbled on reddit before. Basically there’s a cabin in Redcliffe (Hinterlands map) which has a chest which contains Fade-Touched Obsidian as one of its contents. The “guaranteed” part will come into play if you leave one item from that chest (e.g. you would pick up the fade-touch obsidian and other items and leave the gold behind). When you zone out the map (or go in/out the Gull & Lantern tavern north of the village), and zone back in again and check the chest, the fade-touch obsidian will still be there for you to loot again. Remember just to leave the gold (or other 1 item).
The cabin, however, will require Deft-Hands, Fine Tools inquisition perk unlocked.
No Deft Hands, Fine Tools Perk Unlocked?
If you don’t have the deft hands perk unlocked, the second location for a guaranteed fade-touch obsidian is on the Dragon area in the hinterlands. You may need to kill the dragon first since he’ll certainly attack you once you get near the loot.
The loot is located behind where the dragon is standing by, specifically, on a boat.
Just like above, you want to pickup the Fade-touch Obsidian and leave 1 item behind so that the fade-touch obsidian will “respawn” once you visit it again (requires zoning out the map).
Fade touch obsidian gives you +3 guard on hitting the enemy. The higher tier for this is fade-touch Silverite, which gives +5 guard on hit.