Once per area, Tatianna may return to town at the start of the Overlord's turn without using a glyph. The rules of the page 9 say: The overlord may play cards that say “Play at the start of your turn” any time before activating his first monster for the turn.
If I read this and the FAQ about this, I think that spawning a monster is "at the start of the overlord's turn", isn't it? So, could Tatianna return to the city after generating monsters, as it is the start of the overlord's turn yet? If the overlord chooses not to spawn, and then Tatianna uses her ability, could the overlord change his initial decision?