The Tengen NES games two-page advertisement scan from Video Games & Computer Entertainment Magazine April 1989.

I say “Mostly” Legal because the history of Tengen is slighly confusing. They did release Gauntlet, Pac-Man and RBI Baseball for the NES while under a legal NES licensing agreement between 1987 and 1989. Their version of Tetris, which I never played, is routinely considered to be a superior version to the officially released version. I say offically released because Nintendo successfully negotiated with the Russian govenerment to release console versions while Tengen thought it had licensed those from Mindscape.   The Tengen version was pulled from the shelves after selling only a few thousand copies.

They went on to try and attempt to reverse engineer the NES lock-out chip and suffered a loss to Nintendo in the legal relm.

Tengen was the console arm of the Atari Games, not Atari Corp. Atari Corp was the console and computer division that was purchased from Warner by Jack Tramiel in 1984. Atari Games stayed with Warner and had to choose a new name when they went into the Console game business.  They chose Tengen as it was also from the same game, “Go”, that the name Atari comes from.  Tengen released very good games for almost every 1990’s console and computer system. They ported some great Atari arcade games such as Klax, Stun Runner, and Hard Drvin’ to these systems before being re-absorbed into Warner to become Time Warner Interactive.