Hero must survive
From Staredit Network Wiki
Hero must survive is a mission objective typically found in Starcraft missions.[1]
As hero units play an important role in the storyline, they cannot be allowed to die. Death of a hero unit results in defeat for the player.
This gives a player an extra activity to do; they must keep the hero out of danger. In some cases, this may render the hero unit "too awesome to use"[2], so often the best missions involving this objective are extremely difficult without using the hero but possible with the hero's assistance.