None of these will appear until after the airship is obtained.

• Dark Valefor - In front of the village of Besaid. As you enter, a man will appear and summon him.
• Dark Ifrit - Go to the final area of the Sanubia Desert; near one of the quicksand pits is a woman looking for her daught. When prompted to search, go north an area.
• Dark Ixion - In the Thunder Plains, go to the northern half of the area. You’ll see a man standing below one of the lightning rods; speak with him.
• Dark Shiva - Simply return to Macalania Temple and speak with the Guado summoner as you approach.
• Dark Bahamut - In the Zanarkand Ruins, in the room where you fought Yunalesca and where the Sun Crest is found.
• Dark Yojimbo - After having gotten Yojimbo, go to the end of the Cavern of the Stolen Fayth and to the Fayth itself. Things become a bit tedious then, since you’ll fight the guy five times; just never leave the Cave and never use the teleporter (unless you need to).
• Dark Anima - Go to the cave atop Mt. Gagazet. Go to the room where you threw Wakka’s Blitzball at the thingy that had two shields around; repeat this there and win to fight this guy.
• The Dark Magus Sisters - Head to Mushroom Rock Road’s entrance area; you’ll see a woman speaking with a child there. Flee from them; as you go along running away, you will be caught up to by the summoners. If you make it to the other side without being caught, you’ll fight the Magi one by one; otherwise, altogether.
• Penance - Defeat the other Dark Aeons and you can simply choose him from the NavMap on the airship.