Dungeons and Dragons, commonly known as D&D, is a fantasy tabletop roleplaying game published by Wizards of the Coast LLC. The fifth edition of the game is featured on Critical Role and many specials.
The original version of Dungeons and Dragons was originally designed by Gary Gygax and Dave Arneson and published in 1974 by Tactical Studies Rules, Inc., and since then by Wizards of the Coast LLC starting in 1997. The fifth and current edition of D&D was released in 2014. 
A game of D&D requires one Dungeon Master and at least one player. The Dungeon Master (DM), also called Game Master (GM), functions as the planner, narrator, and referee of the game.
The Dungeon Master describes a setting and/or an event, then the players describe the course of action that they wish to take. The DM may ask them to roll a dice, then narrates the outcome. Players are also encouraged to roleplay and interact with each other and Non-Player Characters, or NPCs, to advance the story.
Players must roll dice to determine the outcome of the action they want their character to attempt. The final number can be influenced by the character's skill statistics, abilities, items, and outside forces determined by the Dungeon Master. Modifiers can be added or subtracted from the roll, or the dice can be rolled with advantage (rolling twice and selecting the highest number) or disadvantage (rolling twice and selecting the lowest number).
Different dice are used for different actions. The twenty-sided dice, or D20, is the most commonly used. Players also roll dice with four, six, eight, ten, and twelve sides. Dice sets often include a second ten-sided dice, called a percentile dice, with decimals instead of digits and used with another D10 to roll a percentage.
Rolling the highest possible number on a dice is called a critical success, and rolling the lowest possible number, one, is a critical fail. This means that the action taken automatically succeeds or fails in combat, no matter the total number achieved with modifiers. When trying to land a hit on an enemy, rolling a critical success means that the damage dealt is doubled.
If a dice rolls off the table, hits an object, or is cocked (meaning not completely flat), re-rolls are usually allowed, although this depends on the Dungeon Master.
A character's race will give them certain bonuses, called racial traits; for example, elves are immune to sleep magic, tieflings are resistant to fire, and humans know an extra language. Some races can see in the dark, while others don't. Race also affects ability scores.
A character's class determines their playstyle. Some classes, like bard or wizard, are mainly spellcasters; some classes such as barbarian or fighter don't cast spells and focus on physical strengths; and some classes are a mix of both, like paladins or warlocks. Each class has different rules and provides different abilities to unlock while leveling up. Each class is divided into subclasses, which the player can choose from. They offer a variation in abilities.
Characters can have more than one class. This is known as "multiclassing". The rules for multiclassing are complex and depend on the classes chosen. Essentially, the character has a different level in each class, and adding them together determines their player level.
In Critical Role, characters level up after reaching a milestone determined by the DM. When leveling up, characters gain access to new abilities, new spell slots if they are a spellcaster and their maximum health increases.
Information related to a character is listed in their character sheet, the primary tool used by players. The sheet contains information about a character's appearance, backstory, and inventory, but most importantly their statistics.
Abilities and skills
The main abilities used in D&D are Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma. Player-characters scores are typically set between 0 and 20 and determined by dice roll, and may be affected by a racial or class bonus. The ability score determines the ability modifier, which is the number added to rolls. If the score is above 10, the modifier is positive; if the score is below 10, the modifier is negative.
In some cases, players will have to roll a saving throw. A saving throw gives players a chance to negate some damage or avoid a negative outcome.
Characters also have skill scores. Skills are used in specific situations: for example, Investigation is used when looking for something, and Survival is used when trying to track prey.
Ability scores and skill scores can also be influenced by the player's proficiency modifier, which means that they get a higher score if the character is considered proficient.
Characters have different proficiencies based on their race, class, and background. This includes the languages they know, the tools and weapons they know how to use, and the types of armor they can wear.
Combat in D&D is turn-based. Each round of combat lasts six seconds, and every character involves acts one after the other. To determine the turn order, players and NPCs must roll for initiative, and go from highest to lowest. On their turn, a player or NPC can do three things: one action, one bonus action, and one movement (in whatever order they chose). Once per turn, they also have access to one reaction.
Outside of initiative, i.e. while not in combat, players can freely use actions, bonus actions, and movement.
The action can be used to cast a spell, attack, dodge, grapple, disengage, etc. It's possible to forgo using an action and instead dash, which doubles their movement speed. It's also possible for a player to hold an action until their next turn when something specific happens, like when an enemy enters their line of sight, but if the trigger doesn't happen, the action is lost.
The bonus action is a secondary, more restricted action. Certain attacks and spells can only be done as an action, but players sometimes have access to bonus actions. For example, rogues can use a bonus action to disengage, and some spells can only be cast as a bonus action.
A character's movement is translated in feet. It determines how many squares they can move on a map, with each square being 5 feet wide. To hit another character with a melee attack, the player must be in a square adjacent to them. When leaving a square adjacent to someone else, it provokes an opportunity attack from them, unless the player took the Disengage action before moving.
Most player-characters have a movement of 30 feet, but this number can be different depending on their race, class, and level. Beauregard, a level 16 monk, has a speed of 50 feet.
Once per turn, players can react to something happening on the battlefield if they have a reaction available. Some spells can be cast as a reaction, such as the Featherfall spell, as well as specific abilities.
To attack with a weapon, players must roll a d20 to determine if the attack hits or misses. If it hits, they can roll another dice (depending on the weapon) to determine the damage dealt. Some spells also require a roll to hit.
A character's armor class, or AC, determines if they get hit when an attack targets them. If the attack roll is equal or higher than their armor class, they are hit. Armor class is affected by a character's dexterity and the armor they wear.
Also called health points or HP, hit points are a number representing a character's health. If a character loses enough HP to be brought down to zero, they become unconscious. While unconscious in combat, a player must roll Death Saves with a d20. Rolling 10 and over is a success, and under 10 is a fail. If they roll three successes, they become stable at 0 hit points, but if they roll three fails, the character dies. Typically, enemy NPCs do not roll death saves and are considered dead at zero HP, unless it was specified that the attack was nonlethal.
Spellcasters have access to spell slots or a maximum amount of spells that they can cast each day. When they level up, they gain additional spell slots.
Each spell has a base level, and can only be cast using a spell slot of the corresponding level or higher. For example, a first-level spell can be cast using one of the spellcaster's first-level spell slots, but it can also be cast at levels 2, 3, and so on. A third-level spell cannot be cast using a first or second-level spell slot, but can be cast at levels 4, 5, and so on. Spells often gain in power when cast at a higher level.
Cantrips are spells that don't require spell slots to cast. They are very simple spells that can be cast an infinite amount of times.
Not everyone can use every spell. Spells are often associated with one or multiple classes, and sometimes races. For example, the spell Revivify can only be cast by a cleric or paladin.
Some classes, like wizards and sorcerers, have a small list of spells that they know and can cast every day. Other classes, like druids or clerics, have access to a very large library of spells, but only have access to a finite amount at a time and must choose which spells to prepare every morning for that day. In both cases, players can cast more spells per day as they level up. Half-caster classes, like warlocks, get access to higher spell slots at a slower pace than full casters.
Some races and classes have abilities that let them cast specific spells without expending a spell slot, with some restrictions. Some classes also have access to certain spells every day and don't count against the number of spells they can know or prepare.
Character abilities, spell slots, and hit points can be regained by resting. A short rest is at least one hour long, and players can roll hit dice to regain hit points. They have access to a number of dice equal to their character's level, and the size of the dice is determined by their class. Some abilities can be regained with a short rest, but not spell slots (with a few exceptions). A long rest needs to be at least eight hours long and lets players regain all of their hit points, abilities, and spell slots.
- Gary Gygax, one of the creators of D&D, appeared as an NPC in Critical Role One-Shot: Thursday by Night.
- Jon Michaud (November 2, 2015). The Tangled Cultural Roots of Dungeons & Dragons. The New Yorker. Retrieved on June 10, 2021.
- Larry Frum (May 19, 2014). 40 years later, 'Dungeons & Dragons' still inspiring gamers. CNN. Retrieved on June 10, 2021.
- Mighty Nein character sheets by CritroleStats