The Resonating Crystal allows you to test out new heroes without having to level them.
The way the Resonating Crystal works is that your five best heroes will be placed in a Pentagon. You can then choose any hero that you didn’t level to instantly turn them to the same level as the lowest leveled hero in the Pentagon.
This gives you the flexibility to try out new heroes and see if you want to replace someone from your current roster without having to spend 20 diamonds to reset.
You start off with having 2 free slots to put any hero in there. You can remove them at any time and they will revert back to level 1. It will then have a 24 hour cooldown before you can place a different hero. Using 100 diamonds allows you to skip the 24 hour cooldown.
You can unlock more slots with Invigorating Essences from AFK rewards after finishing Chapter 5 Story Mode.