Find: Summon Phoenix Materia.

Head to Fort Condor. Enter and climb the rope. Climb all the ladders and the stairs to the top. Talk to the man looking out the window here. He sets you up for the battle.

After the battle (if you win), the egg hatches and the mother condor dies. Go outside to check it out. Grab the Summon Phoenix Materia, and watch the baby condor fly away. Go back inside and down a level. Talk to the man sitting at the table in order to get the Huge Materia, then leave Fort Condor.