Climb the mountain near Thamasa, meet the espers and return to the village.
Walkthrough
From Thamasa, take the mountain route west. Explore the caves and the chambers with the statues, watching for exits in the rock walls. The party reaches the espers’ gathering place after a battle with Ultros.
When 
Relm enters the fight, use the prompted action to advance it. After the conversation at the gathering place, return to Thamasa for the Imperial betrayal and the scenes involving General Leo.
The Blackjack becomes the staging point for the assault on the Floating Continent. This is a good time to recruit 
Mog if you want him early, teach useful magic, shop and make a separate World of Balance save.
Checkpoint
Relm’s action changes the Ultros encounter at the mountain. Afterward, the Thamasa story advances quickly toward the Floating Continent. Before initiating the airship assault, leave the village, shop and complete desired World of Balance errands while their locations remain accessible.