Half-Life: Decay - Gameplay

Gameplay

As is the case with the other games in the Half-Life series, Decay is a first-person shooter. Like the original title it is based on, Decay requires players to engage in combat with hostile non-player characters and complete various puzzle solving tasks to advance through the game. However, Decay differs from Half-Life and its first two expansion packs, Opposing Force and Blue Shift, in that it is designed for cooperative multiplayer gameplay. This requires players to work together to progress through the game's levels and complete puzzles as they arise in-game. Although intended to be played by two people in split screen mode, Decay can still be played by a single player. In this case, the player can only control one character at a time, and can switch between the two characters quickly. When not in use, a character has sufficient artificial intelligence to defend themselves, but otherwise does not move from where they have been left by the player.

In Half-Life, players usually fight alone and only occasionally encounter friendly non-player characters who assist them, such as security guards and scientists. While Decay still features levels where this is the case, significant sections in Decay are dedicated to working with friendly non-player characters, usually escorting them to various objectives and protecting them in firefights. An array of enemy characters from Half-Life populate the game, including alien lifeforms such as headcrabs and Vortigaunts, as well as human soldiers sent in to contain the alien threat. The players have access to a limited selection of Half-Life's weaponry to assist them in the game. Although developed after Opposing Force, no non-player characters or weapons from the earlier expansion appear in Decay.

The game is unique in the context of the Half-Life series in that it is the only game divided into separate missions, each with a specific objective to pursue, instead of consisting of a single unbroken narrative. How players perform in each mission is ranked at the conclusion of the level as a grade from "A" to "F". This score is based on each player's accuracy with weapons, the number of kills they acquired and the amount damage they sustained during the course of the mission. Should players successfully complete the game's nine missions with "A" grades on every level, they are given access to a bonus cooperative mission, in which they can play as a pair of Vortigaunts, as well as the ability to play through the PlayStation 2 version of the original Half-Life as a Vortigaunt.