When you roll a 20 for an attack roll, it is a critical hit. This means that the attack automatically hits regardless of the enemy’s Armor Class. Alternatively, In 5th edition Dungeons and Dragons, ability checks do not automatically succeed or fail with the roll of an unmodified 20 or 1. Of course, the rules also state that ultimately it is up to the Game Master to decide how they want to run their game.
Before we dive into the critical success and failure it is good to establish when a player should roll an ability check. In the Dungeon Master’s Guide it describes 2 conditions that dictate when players need to roll. The first is “Is the task so easy or free of conflict and stress that there should be no chance of failure?” The second is “Is the task so inappropriate or impossible – such as hitting the moon with an arrow – that it can’t work?” It goes on to say that if you answer no to both of these questions your player should roll an ability check.
Easy and Free of Conflict
From a purely numbers perspective, being easy means that the Difficulty Class is equal to or less than the skill modifier of a character plus the lowest roll a player can get on a skill check, a 1. This means that there is no way that a player can fail. As a DM, you might decide that a 1 should be a failure, because you feel that there is always a 1 in 20 chance that a character will not being able to do something. Alternatively, as a DM you feel that the characters are heroes and failure in trivial things doesn’t further the story.
“Free of conflict and stress” brings up a common trap for new DMs. If a task is free of conflict or stress the players will continue attempting it until they succeed. Even if their skill modifier means they will need to roll an 18, 19 or even 20. They will roll tell they are successful if there is no consequences to rolling. To reduce the tedium of such a thing, a DM can introduce a consequence to failure or simply assign an amount of time to the task. An example of a consequence to failure would be, you have a locked door that a rogue has the skill to open but fails their first attempt. Maybe this alerts the guards on the other side of the door. Now the group has lost the element of surprise. Maybe there is a guard patrol outside and the longer the group works to open the lock, the greater the chance the patrol will discover them.
Inappropriate or Impossible
The second condition is a little more muddy in a world of magic and fantastical creatures. It reads, “is the task so inappropriate or impossible.” If an untrained peasant with a dagger can land a hit on a Tarrasque with the roll of a 20, would it then follow that if a player rolls a 20 on their Insight(Wisdom) they can read someone’s thoughts. Shouldn’t a barbarian who rolls a 20 on his Intimidate(Charisma) check be able to scare a king into naming them the new king? This question ultimately needs to be answered by the DM.
As they DM you are allowed to say no, you can’t read people’s thoughts without magic, and no you can’t scare the king into making you king. That is unless you want to. In the immortal words of the lead Designer of the Player’s Handbook and Dungeon Master’s Guide, “Live your bliss.” My bliss is giving weight to 1’s and 20’s. I don’t make people roll for things that don’t have consequences to failure, but if I’ve made a player roll, I make a 1 and a 20 a special occasion. It feels great to roll a 20, but to have it have no appreciable effect on a game makes them less special. I don’t have my players roll for things they can’t succeed on but if there is a task that they have rolled a 20 on that means they get a little something extra. Maybe that 20 for a Nature(Intelligence) roll gives them an extra tidbit about the creature that they see. Maybe that 1 for Insight(Wisdom) roll tells the player that the person they were talking to has to be lying to them, because they are out to get them. There isn’t a right or wrong way to play, but it is something to consider when you are running a game.