Your idea is quite interesting; visually, the spell blends perfectly with the image and name of the shamans, but the main problem lies in its implementation. Creating the ability itself won't be difficult, but the damage it deals depends on the number of creatures in the stack. Since there are far fewer Thunderlords, the ability will deal significant damage. If you give it to the shamans, there will be no escaping them. Furthermore, the Daughters of the Earth will become useless.