Hex:

Hex is a hard disable in Dota 2 that turns a hero into a harmless critter and prevents most actions, including attacking, casting spells, using items, and often using mobility skills.

It matters because Hex can instantly stop a key target before they escape or use a save ability, making it one of the strongest forms of single-target control in the game. Unlike a stun, it is usually valued for shutting down both spellcasting and item usage at the same time, which makes it especially dangerous against elusive cores and mobile supports. For example, a Lion can Hex an enemy Storm Spirit before he zips away, then follow up with damage or a stun from the rest of the team.

Related: Stun, Silence, Disable