In Pathfinder 2e the Quickling has an ability called Fade from View which, as a free action, lets the quickling become invisible until it uses an attack, manipulate, or move action. The quickling can't use Fade from View again for 1d4 rounds. To be used it requires the quickling to not have used any attack, manipulate, or move actions in the previous round.
I swear I can't figure out how to use it efficiently and what purpose does such an ability serve. Standard Invisibility doesn't deactivate until an hostile action is used, so it can be used for something, but invisibility that requires you to do basically nothing for a turn and expires as soon as you do something is almost useless. The only scenario where I can imagine some use for such an ability (beside starting combat while invisible, maybe) is if the quickling retreats with all its actions behind a faraway cover and then becomes invisible before someone can get to it while pursuing it. But it's such a niche scenario that it makes me wonder why shouldn't the quickling just run away, considering that it will probably never be caught with its speed.
Best Answer
Situational Purpose, Particularly Outside of Combat
Here's the ability in its full text for reference:
One of the most powerful aspects of this ability is in its duration, as it requires no continued effort to sustain and lasts infinitely until the quickling takes some further action. There's also the free-action activation, but that's somewhat less useful considering the requirements that make it pretty difficult to use in an encounter.
So what can be done with an infinite duration invisibilty?
Ambushes seem like a natural match for this ability, with the quickling waiting in a well-traveled location for some other creature to move directly adjacent to them. The quickling could then initiate a combat using Stealth and Strike at the creature twice before fleeing from the combat with such speed that the attacked creature might not be able to even attempt to pursue.
This idea could even be used multiple times in a series of ambushes, particularly if the creature is trying to follow the quickling or bring it to justice. Each time such a creature gets too close the quickling can make additional Strikes and flee again.
Outside of Ambushes
These are other potential uses but are more circumstantial, in ways that the quickling is less likely to be able to manipulate.