Although considered a True Rune, it does not confer eternal youth while split in two. The rune is destined to come into conflict with the person who bears the Bright Shield Rune. The rune possesses powerful attack magic, able to decimate large groups or focus on single foes.
Twinkling Blade 100 DMG to all enemies. Instant Death (30%) to all enemies.
3
Piercing One 700 DMG to 1 enemy.
4
Hungry Fiend 1200 DMG to all enemies.
Trivia
As part of the Rune of Beginning, the Black Sword Rune and the Bright Shield Rune represent the struggle between Shield and Sword in the creaton myth of the Suikoden world.
The name of the rune might be referencing the Elric saga by Michael Moorcock. In these novels, the main character Elric wields a black sword called Stormbringer which is embedded with red runes. There's also a song by Blue Öyster Cult called Black Blade whose lyrics were written by Moorcock.