This one’s a bit of a cheat, since Nar Shaddaa is technically playable in the BioWare MMORPG The Old Republic. But the game isn’t actually canon (it’s not not canon either, at least until Disney decides if and how they want to incorporate it).
But that doesn’t take away from how the planet is depicted in Marvel’s main Star Wars comic. Its first appearance comes in issue #8, where Luke visits the planet seeking a smuggler who can take him to the Jedi Temple on Coruscant. He runs afoul of trouble after a degenerate steals his lightsaber, and he eventually winds up in the clutches of Grakkus the Hutt, essentially a distant relative of Jabba on steroids.
Think of Nar Shaddaa, appropriately nicknamed “The Smuggler’s Moon,” as the dark reflection of Coruscant. It’s a grimy, dusty, shady town, with a sprawling collection of buildings stacked on top of and against each other. It’s probably my favorite depiction of a new planet so far.
Next: The Paradise Planet