Adeptus Titanicus launched what would become the Epic series. It was launched in 1988, I believe, as a competitor to Battletech, another popular games that had giant mechs striding about dealing death. Adeptus Titanicus had a similar formula of Giant Titans striding about dealing death.
The background setting to the game is the 40k universe set during the Horus Heresy. The rulebook is richly interspersed with atmospheric art and background stories. The rules themselves are pretty detailed but simple to follow and the game had various counters that eased the flow of play – showing Titan damage (lost arms, reactor breaches, etc), objectives, burst markers, etc. The objective system in particular is often cited as one of the best of all the Epic games.
When the game was released, they followed up and released metal and plastic Space Marine Infantry and Vehicles followed by Eldar and Ork titans (gargants) and infantry.