Guild Wars 2

How is it possible to get this when Arah is virtually always under Pact control?

Is it the Magnification Lens you’re referring to?

Game update notes: January 12th.
Astralaria IV—Magnification Lens: This collectible can be earned from the “Ensure that the Pact holds the Gates of Arah” event at the Narthex.

yes, that’s the one.

Thanks, so basically you can get Magnification Lens and Ancient Planetarium Side at the same time?

Sort of. Wiki says the Slide is from the event before that one. The Lens was changed to the other one to remove it from an event that required a failure.

Ancient Planetarium Slide
Defeat the Risen High Wizard