What is a World Rune?

Think of it like the primal forces that shape the earth/Runeterra, condensed into the form of a gemstone inscribed with its correlating rune. They're the biggest cause of Runeterra's upheaval during the Rune Wars (hence the name), as power-mad sorcerors and mages abused the power of the runes to reshape Runeterra and destroy their foes.

They can certainly be used for good-natured ends, but the risks totally outweigh the benefits.

Spooky rocks that can blow entire cities to kingdom fuck.

Reading Ryze's lore gives you a general understanding of what a world rune is.

Really powerful stones that once shaped the Earth it self. Powerful magic stored in one object

Powerful magic glowy rocks.