The Final Strike is a last ditch effort to strike down an enemy. It involves all the energies the person has or has access to in order to deliver what is expected to be a killing blow. This would include pulling from ley-lines and/or nodes which may account for the sheer destructive power this spell. A Final Strike also tends to result in the death of he or she who calls the Strike and, often, only ashes remain.
It would be noted that the Final Strike is almost always employed by a Mage using mage energies. However, an exception to this would be Herald Lavan Firestorm. In Lavan's case, the nature and the sheer power of his Firestarting Gift allowed him to execute a Final Strike that is slightly different than that of a Mage: Lavan simply released what little control he had of his Gift and allowed it to consume as much as it could, including himself.