These can be used to exchange for mounts, wings, or heroes in the first week, and any spares can be exchanged after that.
The first-week event could be seen by clicking on the button showing the Void Sorceress, to the left of your screen. This should give you a few exchange options, including the Void Sorceress hero.
There is also the option to exchange them for ‘Divinity – The chosen one,’ that unlocks a tab in skills and can boost your EXP gain. However, that probably isn’t as useful as the other options, and isn’t worth the cost. While there were plans to expand on that feature, like many other game features it was left incomplete. As such, it may be a bit too expensive for its current benefits.