Nobility is a guild and territory control system meant to bring war mechanics to Minecraft, from popular MMOs like Albion and EVE Online. Nobility includes a guild system, city building & upgrades, territory claiming, and a PvP siege mode.
You can read more about the design of Nobility here.
Development on Nobility is only partially completed. Functional features include:
Guild creation and management
City founding
City upgrades and construction
City build protection
Node claiming
Siege timers
Siege battles, cannons, and structure damage.
City-based crafting system
Administrative tools for the above
Notable features still under development include:
Node resource generation
Siege results
Siege initiation
Database support