There are 2 modes: Classic and Insane. Classic is just this one dude ready to kill you. The Insane mode gives the boss a few abilities. Both spawn End Portals. Good luck.
Classic Fight: /summon EnderDragon ~ ~1 ~ {CustomName:"Objective: Destroy the King",CustomNameVisible:1,Attributes:[{Name:generic.maxHealth,Base:0}],HealF:0,Riding:{id:"Bat",Attributes:[{Name:generic.maxHealth,Base:0}],HealF:0,Riding:{id:"Skeleton",CustomName:"Ender King",CustomNameVisible:1,Equipment:[{id:diamond_sword,Count:1,tag:{display:{Name:"King's Broadsword",Lore:[Earned by defeating King Shorful]}}},{id:diamond_boots,Count:1},{id:diamond_leggings,Count:1},{id:diamond_chestplate,Count:1},{id:skull,Damage:3,Count:1,tag:{display:{Name:"King Ender's Head",Lore:[You can now build your empire. Rule as king!]},SkullOwner:MHF_Enderman}}],DropChances:[2.0F,0.085F,0.085F,1.0F,1.0F],Attributes:[{Name:generic.maxHealth,Base:272},{Name:generic.knockbackResistance,Base:1}],HealF:272,PersistenceRequired:1,Silent:1}}}
Insane Fight: The command is down below... I wish you luck.
Oh and GEAR UP! Do not underestimate the Ender King.... or you will die!