Aghanim's Scepter:

Aghanim's Scepter is a major Dota 2 item that upgrades many heroes’ abilities or adds a new ability or effect to their kit. It is often bought to increase a hero’s impact in fights, improve utility, or unlock a stronger version of a spell.

Because its exact effect depends on the hero, the item is used differently across the roster, but it is generally a high-value mid- to late-game purchase that can change how a hero is played.