Alternity is a science fiction role-playing game (RPG) published by TSR in 1998. Following the acquisition of TSR by Wizards of the Coast, the game was discontinued in 2000 as part of a broader rationalisation of TSR's business holdings, but it retains a small but devoted fanbase. Parts of Alternity as well as TSR's classic Star Frontiers game have been incorporated into the d20 Modern game, especially the d20 Future setting.
Skills are classified into broad and speciality skills. Earning a specialty skill requires an associated broad skill, which requires a character to have sufficient associated ability points. Special skill is further classified into ranks, which affects the skill's scores. Skill scores are presented with the full score, half that score, and one-quarter that score. which represent the numbers needed to achieve Ordinary, Good, or Amazing successes in an action round respectively.
Unlike many other systems, actions are determined by a control die and situation dice. When Gamemaster calls for a roll, player rolls 1 control die and 1 situation die. The control die is always a 20-sided die, while situation die can be a 0, 4, 6, 8, 12, 20-sided die, where 0-sided die means the action only depends on control die roll. Situation die can be plus die or a minus die, in with the value in the situation die is added to or subtracted from control die value. The total of the rolled numbers is checked against character's action, skill, feat, to indicate a success or a failure. Like the games published before 3rd Edition Dungeon & Dragons, rolling low is always better for successfully completing an action.
The type of situation die being used depends on the difficulty of the action. Difficulty is scaled in die types of -d20, -d12, -d8, -d6, -d4, +d0, +d4, +d6, +d8, +d12, +d20, +2d20, +3d20. A character's base situation die is +d4 for broad skill or feat check, +d0 for specialty skill or action check. A minus situation bonus means player uses a larger negative situation die set, while a plus situation penalty means a player uses a larger positive situation die set.
In an action round, a round is divided into 4 phases. Each phase relates to one of the degrees of success that are achievable on an action check: Amazing, Good, Ordinary, and Marginal, in order from the first phase to the last. A hero can attempt only 1 action per phase. Acting orders of characters are determined by a d20 die roll for all participants, which determines the earliest phase in which a character can act. All actions in a phase are considered to occur simultaneously, with the results of those actions being applied at the end of the phase. A character can act in as many phases as it has actions per round.
Depending on how far below the skill score the player rolled, there are 3 progressively better layers of success and 2 levels of failure. An action is determined using this same system, making the game very uniform. Only armor rolls and damage rolls did not use the d20.
Life points, called 'Durability', are categorized into Stun, Wound, Mortal. Stun damage can immobilize a character, but not life-threatening; wound damage can immobilize a character and inflicts 1 stun damage point for every 2 wound damage points received; mortal damage can kill a character, and inflicts 1 wound damage point for every 2 mortal damage points received. Durabilities can be repaired by healing, or:
Designed to be a generic rule set around which a campaign world could be built, it was not very heavily marketed and suffered from mediocre sales which, along with increased focus on the d20 system, led to the discontinuation of the game in 2000.
Much of the content of the Alternity game has been absorbed into the d20 Modern role-playing game. The Dark•Matter campaign is an entire d20 Modern expansion and Star Drive is part of the d20 Future expansion. The Gamma World campaign is an d20 Modern expansion by Sword & Sorcery Studios (White Wolf).
The probability curve created by the addition or subtraction of a d20 and another die is shaped like a plateau, with two straight lines on both ends of the flat region. This is intermediate between the totally flat probability curve rolled by rolling a 20-sided die and the bell-shaped curve produced by die pool systems.