Arcane rune:

An Arcane Rune is a rune that temporarily reduces the mana cost and cooldowns of the hero who picks it up. It is one of the power runes that can swing early fights, let a midlaner clear waves faster, or let a spellcaster cast key abilities again in a fight.

Its value comes from tempo: a hero with limited mana or long cooldowns can use Arcane Rune to spam spells, pressure an objective, or secure kills before the effect ends. For example, a Storm Spirit who grabs Arcane Rune can zip through a skirmish, spend more mana on disables and mobility, and then continue casting without needing to retreat immediately for regeneration.

Related: Bounty rune, Power rune, Rune