Nuclear is a transcendental skill that has been mentioned multiple times. While its specific effect is unknown beyond mentioned to be comparable to nuclear fusion, it has been hinted that it has immense destructive power. Its main user is Agni.
Because Nuclear is a General Transcendental, it can be used by anyone who knows the skill and has enough vigor to use it. According to Currygom, Nuclear is one of the few General Transcendentals that is more powerful than a Unique Transcendental. It's very rare for a general transcendental to be more powerful than a unique one.