Valley of Treasures

This is similar to the dark and light campaigns where you play through the game story level by level and try to get three stars. The main difference is that you can use both light and dark side characters.

So if you have not unlocked the dark side campaigns yes, it still may be worth investing some upgrades into a dark hero as you can play them in this mode of the game.

It's really worth playing through these levels especially if you are stuck on the light side missions as the valley of treasures will also give you a ton of loot to equip and upgrade your heroes giving you a better chance to progress in the main storyline part of the game.

The energy meter is independent of the main game energy too, so if you run out of energy with playing light and dark side battles, and still want to continue playing, then you may well have some unused energy in the Valley of Treasures that can be used up.