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.
• A4 is the Arches of Ulloch, a pair of gigantic archways usable as gates with appropriately tuned keystones. One is untuned right now, while the other is usable to get anywhere in Avernus.
• A5 is home of Warlord Kolasiah and their gang, the Lost Golgari. Their forge contained astral pistons, which they gifted to Solarys for banishing them back home.
• A6 used to be Bitter Breath's war camp, until Bitter Breath was killed by the party. Now I guess it's their war camp.
• 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.
• B5 contains an enormous crater, created when a colossal skull appears to have plummeted to the ground here.
• B6 is an abandoned demon shrine, now ruled by the Soul Collectors, who hold gladiatorial arena matches in a pit there.
• 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 the former site of a gigantic helmet that served as the shop of Uldrak the Tinker. Upon making a deal with Arkhan to obtain Tiamat's blood to free Uldrak, Uldrak returned to his home plane, taking his helmet with him (but leaving behind his stuff).
• 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 used to be the site of a crashed elemental galleon from Eberron, but the party has turned it into a spelljamming vessel.
• 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.