I don't understand the question.
Dark forest is this whole map with Elven ruins, Elven encampment, Dwarven halls, Imp's castle, Warrior's guild,...
And since you're asking this question about Imp's castle, that means you are in the Dark forest.
In Rivertown (1. main map) is the road to Dark forest (most south) and in the Dark forest is Road to Rivertown (most north). Both roads are marked with red flag on your map (press A-key to open map).
Get near the flagged place and you'll be asked "Do you want to go to Dark Forest/Rivertown?".
But which teleporter are you looking for?