Where leylines intersect, a nexus may be formed. Nexuses amplify the flow of magical energy, and may also function as a kind of lens which colors or otherwise alters the nature of this energy. (The islands of Nyroth offer some of the most striking examples of the effects this can have.) Mages can control nexus behavior to some extent, and can use nexuses as focal points for spells comparable in magnitude to rune magic (and debatably classified as such), though with much less attendant danger.
Nexuses are extremely valuable to magic-using civilizations. The Kalamanda incident arose when Demacia and Noxus fought over the site of a newly discovered nexus. This clash of superpowers was a geopolitical catastrophe, but perhaps equally frightening was the potential for arcane fallout as battle-magic raged atop a highly charged and increasingly unstable thaumic foundation. Once peace was restored, Summoners of the League of Legends constructed a Field of Justice known as The Crystal Scar to begin repairing the damage.
By 24 CLE, nexus formation had become a rare event. The restoration of Nyroth triggered a thaumic shockwave that energized Valoran's leyline network, and sparked the creation of the Ironspike nexus in the Ironspike Mountains. A tournament was held, and Zaun won control over this nexus.