Instructions
Travel to Darrill's Tomb, in the southwest. This should be your first mission in the World of Ruin, so you are able to collect the scattered members of your party. Defeat Dullahan at the end of the dungeon and after the cutscene, you'll be rewarded with the Falcon, your new airship.
Travel to Mobliz, at the end of the Serpent's Trench. In this town, you'll find Terra taking care of two small children in a house in town. Once you talk to her and exit, the monster Humbaba will fight your party. Terra will attack on her own and fail. Beat Humbaba and he will run away.
Leave Mobliz, then come back. Since you have the Falcon, the second fight with Humbaba can be triggered. At the end of this fight, Terra will regain her strength and help you defeat the monster and join your party.