Yin Yang Bomb

When light and darkness are used at the same time.

Per the TV Tropes Wiki, this is when a character draws upon two diametrically opposed power-sources or styles, and combines them into their own fighting style or school of magic. Could be light and darkness as mentioned above, or matter and antimatter, fire and ice, order and chaos, chocolate and peanutbutter, etc.

Falls under the umbrella of weapons and wielding tropes and/or magic and powers tropes, depending on what the sources are and how you use them.

Sources

Bibliography

1. TV Tropes

Game and Story Use

• Could be a way to make an antihero or reluctant hero be mechanically different from the other characters.
  • Because of their dark past, they have access to two different suites of power. Maybe a multiclass situation?
  • May also be appropriate for emo/goth/angsty characters.
  • Or, it could be the sign of a well-balanced holistic monk character. True Neutral because they've already mastered the extreme paths and moved on.
  • Probably something of a renegade, as the normal followers of either power centre are unlikely to tolerate the study of the diametrically opposed power.
• This could be the intended plot/resolution of your scenario: In order to defeat the big bad evil guy, the heroes must learn some facet of his power, and successfully merge it with good magic.
  • Might have some potential synergy with body horror, lycanthropy, body snatchers, vampires, corruption, mutation, etc. The PCs are up against some sort of monster that can horribly transform them, but if they survive being contaminated, they can emerge from the condition with greater power and understanding.
• You also have the very literal matter-antimatter collider - or, for magitek, the negative and positive energy plane gate collider.
• For those that fancy a bit of Jung, go all Wizard of Earthsea and have a character chasing their shadow all over the world to integrate it and balance their psyche. Those who merely expel their shadow become a white mage with safe, but limited power, but those who fail to manifest it run the risk that it will overwhelm them and turn them into a black mage at the beck and call of their negative emotions - the long struggle to first manifest the shadow and then re-integrate it allows the grey mage to master the more dangerous parts of magic without being mastered by them.
• Leaving aside the moral aspects - and recalling that in the Eastern tradition Ying and Yang are mutually complementary as well as opposed, rather than being "good" and "evil" (more like "male and female" in many ways), this would also apply well to elemental magic - normally studying fire magic at least severely impedes study of water (likewise earth and air) so an elementalist is limited to two unoppsed elements (Chinese elementalism is even more complicated). So presumably some kind of secret wisdom is needed, perhaps mastery of "void" or "quintessence" before taking on a third element.