After reuniting with Sabin in Tzen, you can check the town of Mobliz in the northeast.
Optional Trip to Mobliz
You can skip this section if you wish and do it later. The reward for going to Mobliz the first time is the Fenrir Esper, which teaches Warp/Teleport, X-Zone/Banish, and Stop. When used as an attack in battle, Fenrir does the move Moon Song/Howling Moon, which casts Image on the party.
To go to Mobliz, you can either walk northeast to go there, or go into the small forest northwest of Tzen to get a Chocobo. Mobliz is on the far right side of the narrow piece of land in the northeast of the current continent.
130. Delta Bug/Delta Beetle
Gravel and Forest:
131. Bloompire/Vampire Thorn
In Mobliz, you can heal up for free in the bed in the Relic Shop. Then go into the house where you saw the child go. Check the clock for an Elixir, then go downstairs and through the door. There will be a cutscene. Go up to where Terra went, and talk to her. Then leave.
Let Phunbaba/Humbaba defeat Terra. Then Celes and Sabin will fight him. He is weak to Poison, so use Bio if you can. Do not use Lightning, because Phunbaba/Humbaba absorbs it.
You will get the Fenrir Esper as you leave. Heal up in the Relic shop.
Your next destination is Nikeah, in the northernmost tip of the narrow part of this continent. You can find a Chocobo Stable in the left side of the forest that is south of Mobliz.