Dark Valefor is one of the Dark Aeons and spawns after you have obtained the airship Fahrenheit. Many gamers encounter Dark Valefor accidentally if or when they return to Besaid Village later in the game. Most then experience a very quick defeat at the hands of the powerful Dark Aeon!
Note that if you are attempting to return to Besaid Village either to recruit Blitzball players or to obtain the 
Jecht Sphere near Besaid Temple you will not be able to get past Dark Valefor without defeating it. There is no way to get by it.
As mentioned above, Dark Valefor can be found in Besaid Village. You will be ambushed by a Yevon Acolyte who will summon the creature to attack your party.
Each of the Dark Aeons, including Dark Valefor, are considered superbosses and are some of the strongest enemies that you can fight in the game. As such, you will need to ensure that your party is fully leveled, has their best weapons and has their stats maximized as much as possible. Refer to the Advanced area of the Tips and Tricks section for additional details:
HP: 800,000
Elemental Affinities: 50% reduction to all elemental damage
Immune to Sensor
Dark Valefor is the easiest of the Dark Aeons to defeat. Most of its attacks will do relatively minor damage with the exception of its Overdrive ability, Energy Blast, which can deal very heavy damage.
Make sure that you cast 
Auto-Life on each of your characters for when Energy Blast comes up. Alternatively, you can have Yuna summon an Aeon who can take the blow for you (which will then KO that Aeon instantly).
You can Steal the following items during the battle:
X-Potion (x2) (common)
Elixir (x2) (rare)
And Dark Valefor will drop the following items when it is defeated:
Dark Matter (common)
Master Sphere (rare)
