At the star-shaped mountain divide into two parties.
Walkthrough
Land on the mountain with the star-shaped opening and divide into two parties for Phoenix Cave. Give each team healing, revival and suitable equipment before entering; an unequipped second team can strand progress.
The cave uses pressure plates and alternate routes. Leave one group on a switch while the other crosses, then switch parties to open the next route. Watch for lava paths and explore accessible side passages before dropping through one-way holes.
At the far end, meet 
Locke and follow the events in Kohlingen. Phoenix joins the magicite collection and Locke returns to the party. Check the equipment on both groups when you reorganize afterward.
Checkpoint
Give each Phoenix Cave party recovery and useful equipment before entering. Pressure plates open routes for the other team, so move the inactive group only when its switch is no longer needed. The Locke event continues to Kohlingen and awards Phoenix after the cave.