Elemental Fusion has two forms.
The first is the basic elemental fusion, where a warrior with double affinity—ice and fire—can fuse the two to form a new type of element.
This method is widely known, and many can do it, provided their understanding of the elements involved is deep enough.
The second is Law Elemental Fusion. This method, like its name, requires one to have some knowledge of elemental law, something that many struggle with.
Every element has its own laws governing it. By understanding this law, one can easily bend the element in however forms they want. However, understanding it is much more complex than one might think.
In fact, the idea of merging two elements to form a new one, with the law as the focus, isn't something that can easily be achieved. So one can only imagine the shock these people felt when they saw that Klaus had fused elements.
Unfortunately for them, even Klaus isn't at the point where he can fuse two or more elements using the Law.
