The Stolen Sigil - Act 4 Side Quest
Start Location: Medea's Grove
Quest Location: Ixian Outlands/Ampelian Caves
Objective: Defeat Queen Ycht'ssk'l and return the Sigil of the First Apoktisis to Semele
The Stolen Sigil is a Side Quest that begins in Medea's Grove, the second town of Act 4. To begin this quest you'll want to speak with Semele who is standing right next to the Portal in town. She asks you to defeat Formicid Queen Ycht'ssk'l and acquire the Sigil of the First Apoktisis.
You can find Formicid Queen Ycht'ssk'l in the Ampelian Caves northeast of Medea's Grove. In order to continue to Soronis, the next area, you'll need to pass through these caves. The queen can be found in the northeastern most portion of these caves, an optional part of the caves which is a little bit out of the way. Use my map screenshot below for an exact location.
Once you've acquired the Sigil of the First Apoktisis return to Semele and speak with her to turn the quest in.