Overskill

An Overskill (口伝オーバースキル), is an emergent use of one's own abilities, after the Catastrophe, in a way that was not designed by the game of Elder Tale.

Ga-tan calls it the "Skill of the Giants"; given that she is native to the world of Theldesia, it is possible that Overskills are something that already existed.

Overview
Overskills are discovered through experimentation, but are also often difficult to master without training and research. Potentially, these skills can be categorized as Continent-level or even Global-level Magic, depending on how they are applied and what impact they have on the world of Elder Tale. Overskills such as Shiroe's Contract Art Ceremony or Magical Engineering have already caused societal upheavals among the People of the Land.

Combat-usable Overskills register an entirely new skill into the system, prompting a notification to appear saying what skill had been created.

Trivia

 * According to one of Mamare's Twitter notes, Real Food Preparation creates food that can cause weight gain, unlike menu-crafted food.
 * The Japanese kanji name for Overskills, 口伝 (kuden), follows the naming pattern of regular Skills, and is the uppermost skill level in martial arts where the school's secrets, mysteries, and best techniques are only passed down orally to keep them from spreading.
 * Two other terms used for Overskills include "skill of the great ones" (巨人の技) (by Ga-tan) and "boundary-destroying art" (崩界技).