Sicarus
Sicarus is the current Daemon World of everyone's favourite Chaos crusaders, the Word Bearers. Sicarus for all intents and purposes, is 40k's City of Dis, a giant city world made from hundreds of thousands of cathedrals dedicated to heretical faiths.
Overview
Outside of arguably Sortiarius, Sicarus is one of the most 'cosmopolitan' of Daemon Worlds (If you could equate mass impalement and daemon fucking as 'cosmopolitan'). As such, it's kind of like a Daemon-Hive-Shrine World in classification.
Due in part of this, beneath a roiling sky of fire and blood, the geography of Sicarus is a world completely covered by vast sprawling cathedrals, temples and monuments dedicated to the worship of the Ruinous Powers. It's like Lorgar's wet dream made manifest. Spider-legged cranes and a metric fuck ton of slaves tirelessly toil in a never-ending task to raise more structures for KHAAAAAAAAOOOOOOOOOOOSSSSSS! Level upon level atop the existing older crumbling edifices and cathedrals, producing obelisks and spires many kilometres high. As a result, the majority of buildings are subterranean; a labyrinthine warren of interconnected structures devoted to the worship of Chaos.
The seat of the Dark Council is the Basilica of the Word — an immense cathedral fortress crowned with hundreds of five-kilometre high barbed spires, each studded with jagged spikes, upon which are impaled countless living sacrifices. Also on the Daemon World is the immense Templum Inficio, the home of the Daemon Primarch Lorgar who is currently hiding beneath his blankie and shitting the bed 'cause Corvus Corax is waiting outside of the window for a goold'ole knife fingering.
The bodies of fallen Dark Apostles are buried in Sicarus's extra heretical soil.