Tenchu: Stealth Assassins

Tenchu made staying unseen safer than fighting, alongside two other 1998 games. Tenchu arrived in August 1998, before Metal Gear Solid in October and Thief in November. It cast the player as a ninja in pitch black villages who strikes from the shadows.

Before 1998 stealth was a rare side idea. Sega's 005 had flashlight guards and Castle Wolfenstein had disguises. These three games set rules still used today.

Start missions with unaware guards so players have a state to protect. In Tenchu, Metal Gear Solid and Thief, guards patrol with no knowledge of the player's position or existence, and the core goal is to keep it that way.

Quote: "all of the games have you start missions in a neutral, non-hostile state"

Tenchu's Ki Meter removed guesswork about whether a guard could spot you. The Ki Meter changes colour with guard awareness and also shows a number for how close the nearest guard is, so players can plan a route instead of guessing.

Metal Gear Solid's Soliton radar and Thief's Light Gem solve the same problem. Quote: "it removes the ambiguity of whether or not you're about to be spotted"

A suspicious middle state forgives small mistakes without a full alert. Tenchu's guards investigate odd sights and sounds before attacking, and give up the hunt if the player hides.

Being able to recover means players avoid reloading a save or killing everyone. Quote: "guards have a middle state where they're merely suspicious and will investigate suspect sights and sounds"

Let players steer guard behaviour, like Tenchu's poisoned food trick. Because guards react to what they see and hear, Tenchu lets players trick them into eating poisoned food to create openings.

Metal Gear Solid uses wall knocking and Thief uses noisemaker arrows for the same purpose. Quote: "manipulating guard AI to your own needs"

Tenchu's grappling hook turned rooftops into the safe route above guards. A freely aimed grappling hook and a generous jump let players move above enemies unseen, then drop down to silently dispatch them.

The Arkham series later used the upper areas of levels the same way, to plan moves and hide. Quote: "using the verticality of levels to stay unseen"

Tenchu's cinematic stealth kills spread to later games, including Sekiro. Its stealth kills were taken up by later games, and publisher FromSoftware used the series as inspiration for Sekiro: Shadows Die Twice.

FromSoftware, makers of Dark Souls, had planned a new Tenchu game, and Miyazaki's Sekiro was set to feature stealth elements.

New 3D and CD technology made Tenchu's kind of game possible. The shift to 3D and CD hardware enabled games about hiding from enemies, the same tech wave that produced Grand Theft Auto and Parappa the Rapper.

Thief needed CD quality audio and advanced guard AI, and Kojima returned to Metal Gear only because polygons let him show a hiding player's view.

The game that popularises a genre is often not the one that invented it. Metal Gear 2 already had radar, noisy grates, suspicious guards, wall knocking and a reset to neutral, but it ran only on the MSX2 in Japan and barely influenced anyone.

The commercial and critical success of the 1998 games spread these rules across the genre, much as Doom's imitators were called Doom clones though Doom did not invent the shooter.

Thief let players reshape levels to suit their approach. Thief's open, interconnected levels had many routes, and water arrows put out candles to make darkness while moss arrows made silent tiles.

On higher difficulties Thief forbids killing guards. Hitman and Dishonored share its open level design.

Metal Gear Solid showed a countdown until guards reset to neutral. After being spotted, a timer showed how long to stay hidden before guards returned to normal, and its radar jammed while players were detected.