Summary:

Urban Corruption delivers one of the hardest survival experiences in modded Minecraft. Initially, players will spawn into an abandoned city crawling with monsters. To survive, the player must seek refuge in the abandoned buildings and protect themselves from the hordes of monsters. Many different turrets and walls along with powersuits and body modifications can be used to protect players from the evil monsters.

To generate in multiplayer worlds:

Set the world type to lostcities_bop and generator settings to {"profile":"nodamage"}

See screenshots of modpack here: