"Doom" has been considered the most important first-person shooter ever made. It was highly influential not only on subsequent shooter games but on video gaming in general, and has been made available on almost every video gaming system since. Multiplayer gaming, which is now integral to the first-person shooter genre, was first successfully achieved on a large scale by "Doom". While its combination of gory violence, dark humor and hellish imagery garnered acclaim from critics, these attributes also generated criticism from religious groups and censorship committees, with many commentators labelling the game a "murder simulator". There was further controversy when it emerged that the perpetrators of the Columbine High School massacre were fans of the game; the families of several victims later unsuccessfully attempted to sue numerous video game companies - among them id Software - whose work the families claimed inspired the massacre. In 1994, Raven Software released "Heretic", which used a modified version of the "Doom" engine that allowed for vertical aiming, an inventory system to store and select items, and gibs.