Ingen's invisibility potion

In the caves on the island used by gnolls to summon demons, the party wandered into a chamber filled the bones of people who died violently, presumably those killed by the wolf demon. Anrel found a satchel that he recognized as having belonged to Ingen Persson, a soldier formerly under his command who he'd believed died seven years earlier. Inside the satchel, Anrel found items that he also recognized as belonging to Ingen: a leatherbound journal, a set of thieves' tools, and a vial of nearly transparent liquid, all of which Anrel picked up.

While hiding from ape-bodied, bat-faced, boar-sounding demons behind an illusory rockface cast by Grokko, Anrel used a taste test to identify the transparent liquid as a potion of invisibility.

Not long thereafter, Grokko consumes the potion to scout the entire north-western wing of the temple of Zaheer. He almost immediately steps on a pressure tile that activates a poison dart that strikes him, making Vichthart believe Grokko is in danger (due to their Warding Bond). Vichthart and Anrel rush to Grokko's aid, which annoys Grokko because it needlessly squanders the stealth they were aiming for.

After Grokko confirms he's safe, he continues scouting, and fortunately the misunderstanding doesn't seem to have alerted anything to their presence. He follows corridors to locate the last two throat rooms as well as a jail holding several captives (including Celia and Preston).