Zack is a minor character in Nostalgia Critic and a one-shot villain who appeared in the Critic's third installment of reviews based around old commercials - he is a parody of a child character known as Zack the Lego Maniac, who the Critic (somewhat tongue-in-cheek) envisions as having grown up to become a mentally unstable 35 year old man living in his grandmother's house, working on bombs and making a list of bullies he seeks to kill.

Zack does not appear often but is noteworthy for killing the singing "announcer" that plays during his sketch as well as making an appearance at the very end of the Critic's review - blowing up the Critic's home (though the Critic mysteriously survives, as is common in parody works, where humor overrules common logic).
The Nostalgia Critic Short - "Zack The Psycho Maniac"00:47

The Nostalgia Critic Short - "Zack The Psycho Maniac"

The famous parody which started the "Zack" storyarc