D&D 5E (2014) Where is Lucille in BG:DiA?

[plisnithus8]

Quick question:

Where is the fat white pit fiend named Lucille in the Baldur’s Gste: Descent into Avernus book?

 

[Leatherhead]

Underneath Elturel, hanging out near the River Styx. You can encounter him twice, once on the way out into Avernus, and once when you try to free Elturel.