After less than a year on the job and amid tough times for music video games, Dan Rosensweig, head of Activision Blizzard Inc.’s Guitar Hero unit, unexpectedly quit Tuesday to become chief executive of online textbook-rental company Chegg.com.

The former Yahoo Inc. chief operating officer and Quadrangle Group partner, who is well known and respected in Silicon Valley and media circles, was hired by Activision Blizzard CEO Bobby Kotick in March to bolster Guitar Hero, one of the video game publisher’s three key brands, along with World of Warcraft and Call of Duty.

Rosensweig, who got close to Activision’s chief while Kotick served on the Yahoo board, was expected to rise in Activision’s ranks.

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