Arcane Labyrinth

The Arcane Labyrinth is found in the Dark Forest, and is unlocked early in the game after you have played through some of the normal campaign levels. When it has been become unlocked, you will be notified by the game.

The Arcane Labyrinth resets every few days, and you have until the timer runs out to get through all of the available levels of the Labyrinth to get the final rewards which are super useful to help you unlock the likes of Nemora, Estrilda, Khasos, Shemira, Skreg, Seirus, Thoran and Raine.

This list will probably change from time to time, but it is correct in my version of the game at the time of writing (Oct 2019)- Basically you spend your Labyrinth Tokens in the Labyrinth Store to unlock these heroes.

To get through the Labyrinth you'll need a well leveled up team, but you can boost your team by selecting a pathway that offers you free heroes to join your squad while you are in the Labyrinth. This is really useful as once your heroes are dead, you may not be in a position to revive them, so you'll need these replacement heroes to keep on fighting in the Labyrinth.

You'll also collect a load of relics, these however only last for the duration of that Labyrinth, once it resets they are gone, and also they are not usable outside of the Labyrinth.