Map of Avernus

Some additional info on places you've been or know about:

• The green blob that stretches across part of northwestern Avernus is the Pit of Shummrath, a pit fiend punished for his crimes by being converted into a valley full of sentient green slime into which the River Styx flows, slowly eradicating his memories.
• A5 is home of Warlord Kolasiah and their gang, the Lost Golgari. Allegedly they have a forge that may contain astral pistons.
• A6 is where Smiler says Bitter Breath's gang -- his former underlings -- have holed up.
• B1 is a tower claimed by Arkhan the Cruel, the Chosen of Tiamat. Uldrak wants to trade his orb of dragonkind for the reliquary of Tiamat's blood around Arkhan's neck.
• B2 is a Ferryman's port, where people can pay to go across, or more rarely up/down, the Styx.
• B3 is the summer home of Iggwilv, better known as Tasha, the daughter of Baba Yaga herself. Her clone-sister Natasha bears a grudge, claiming she is actually the inventor of Tasha's Cauldron of Everything.
• C5 is a Hellrider crypt, built by Lord Olanthius due to being bound to Zariel even after his death.
• D2 (south of the river) is the base of Warlord of Avernus Carol d'Vown, aka "that bitch Carol." There may be a way across the river here.
• D4 is the Bloody Crescent, a gigantic naturally-formed half-pipe made of rock where the party met Smiler.
• D5 is a gigantic helmet that serves as the shop of Uldrak the Tinker. Uldrak had astral pistons in stock once when Mad Maggie needed them, and has another pair he will trade to the party if they get Tiamat's blood from Arkhan.
• E4 is the foundation of the Tower of Urm, surrounded by a lake of boiling blood; Mordenkainen himself is said to teleport his tower to Hell sometimes and recruit fiends for various information and goods-gathering.
• F2 (it is not especially clear on which side of the river) is the Purple City. Not much else is known about it.
• F4 is a gigantic bloody cyst growing out of the ground itself.
• F5 is a 100-foot-tall spear stuck point-first into the sands of Avernus; the skeletal hand of whatever wielded it appears to still be gripping its gigantic leather-bound hilt.
• G5 is the site of a formerly-highly-guarded bridge across the River Styx. Both sides were drowned out by Solarys redirecting the River Styx into the towers. Also, Plagueshield Point, a former Material Plane drow stronghold inside a stalagmite, is at the bottom of the river underneath the bridge.
• H2 contains an Avernian forge run by Bel, the former Lord of Avernus. Avernian forges may be a source of astral pistons.
• H3 is Elturel. It is attached to mechanical devices on each side of the river by gigantic chains.
• H5 is a crashed weird galleon of some kind. Whatever it is, it's not from Nirvana.
• I4 is Fort Knucklebones. Home of Mad Maggie and her gang.
• I5 is the Spawning Trees. Krikendolt remains here with his pet abyssal chicken Earwax.
• I6 is the tower formerly known as Shadowswimmer Tower, which now technically belongs to the party as its owner, Respen Shadowswimmer, was voluntarily banished back to the Material Plane.