Not sure if you need help on part of it, or the whole thing.
Unit enters region is the correct event. Make a region in the terrain editor, and reference it in the event.
You'll want to add a condition. If there's only one dehaka, you can use "unit type of unit" then choose triggering unit is dehaka.
In actions you'll want to turn the trigger off first (so it only happens once). Next you can send the dialog.