The following command will summon an enchanted sword that when killed will rarely drop an "Enchanted Sword" but BUT CAN ONLY BE SUMMONED AT NIGHT!!!:

/summon Skeleton ~0 ~1 ~0 {Equipment:[{Count:1,id:diamond_sword,tag:{Unbreakable:1,display:{Name:Enchanted Sword,Lore:This sword is enchanted to live, breathe, and b]KILL![/b]]},ench:[{id:18,lvl:500},{id:20,lvl:500},{id:19,lvl:500},{id:21,lvl:500},{id:16,lvl:500},{id:17,lvl:500},{id:34,lvl:500}]}},{},{},{},{}],CustomName:Enchanted Sword,CustomNameVisible:1,Attributes:[{Name:generic.maxHealth,Base:100},{Name:generic.movementSpeed,Base:0.3},{Name:generic.attackDamage,Base:15},{Name:generic.followRange,Base:10},{Name:generic.knockbackResistance,Base:2}],DropChances:[0.1F,0.0F,0.0F,0.0F,0.0F],ActiveEffects:[{Id:14,Amplifier:1,Duration:999999},{Id:8,Amplifier:1,Duration:999999},{Id:1,Amplifier:1,Duration:999999}],Silent:1,PersistenceRequired:1}

I repeat: ONLY SUMMON IT AT NIGHT!!!

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(Credit to http://www.minecraftupdates.com/summon-command for the awesome command generator!)