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To keep it short, the D&D Dungeon Master's Screen has certain Difficulty Levels with the respective DC. The issue is that there are two Difficulty Levels namely Very Hard and Nearly Impossible whose DC is greater than 20.

My question is; If you are trying to, for example, lockpick a very high-difficulty lock that requires a DC 25 and you get natural 20, do you lockpick it as it is "Natural Success" or do you have to have a +5 or higher modifier with thieve's tools as well? (My question applies for anything that may require a DC higher than 20, I set an example with the lockpick)

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  • \$\begingroup\$ @Zibbobz please, do not answer (not even partially) in the comments. The comments are for clarifying and improving the post only. Answers in the comments section will be removed by the community, so please, if you have a valid complete answer move it to the answer section. See this post for explanation. Thanks! \$\endgroup\$ – Rubiksmoose Jun 18 at 15:12
  • \$\begingroup\$ Are you also curious about saving throws? Saving throws use DCs but aren't Ability Checks. \$\endgroup\$ – Captain Man Jun 18 at 15:23
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There is no automatic success for ability checks

Rolling a natural 20 only guarantees success when attempting an attack in 5e.

For skill checks, a natural 20 essentially means the pinnacle of what you are capable of doing naturally. So if the DC of a lock is 25, and your modifier on the roll is only +3, the lock is impossible for you to open without some sort of outside help that grants you a bonus (i.e. Bardic Inspiration, some sort of magic spell, etc.)

The PHB (pg. 174) further details this:

To make an ability check, roll a d20 and add the relevant ability modifier. As with other d20 rolls, apply bonuses and penalties, and compare the total to the DC. If the total equals or exceeds the DC, the ability check is a success the creature overcomes the challenge at hand. Otherwise, it’s a failure, which means the character or monster makes no progress toward the objective or makes progress combined with a setback determined by the DM.

Consider the notion of someone attempting to jump to the moon, which could be considered impossible. You could set a DC for such a feat if you were so inclined, but even if a 20 were rolled, the character would never meet the DC.

Otherwise, 1 in every 20 children would succeed when they tried. And the moon is a terrible place to raise a family.

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    \$\begingroup\$ The Moon is indeed a harsh mistress. ;) \$\endgroup\$ – KorvinStarmast Jun 17 at 14:39
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    \$\begingroup\$ Similar to Mars, it's cold as hell. \$\endgroup\$ – Verdan Jun 17 at 14:47
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    \$\begingroup\$ @iaminsensible That is an attack roll, not an ability check. Attack rolls have a hit on a natural 20 regardless of AC. See the citation in my answer. There are three primary uses of a d20 cited in the rules: Attack rolls, ability check rolls, saving throw rolls. Only the attack roll has the auto hit on a 20, and an auto miss in the 1. Saves and ability checks do not, unless we look at the special case of the death saving throw: also cited in my answer. \$\endgroup\$ – KorvinStarmast Jun 17 at 15:04
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    \$\begingroup\$ ".. the moon is a terrible place to raise a family." Unless you are a Necromancer, in which case you have uncounted corpses from the children that succeeded, with very little decay. And really, how bad can a Necromancer be if he's an adoptive parent? \$\endgroup\$ – cpcodes Jun 17 at 20:33
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    \$\begingroup\$ @cpcodes that gives a whole new meaning to "raising a family". \$\endgroup\$ – Erik Jun 18 at 12:02
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The PC needs modifiers to raise that 20 to 25 or higher

If you are trying to, for example, lockpick a very high-difficulty lock that requires a DC 25 and you get natural 20, do you lockpick it as it is "Natural Success" or do you have to have a +5 or higher modifier with thieve's tools as well?

The core rules put Proficiency into play, as well as Expertise, by not having "critical success" for anyone trying a difficult task. For very, very hard tasks (above DC = 20) the "anyone can try it" approach will typically fail. For a PC with a skill proficiency, or double that (expertise doubles a skill proficiency) there is a chance for success.

  • It is worth keeping in mind that anyone can try most ability checks even if not proficient. They'll generaly have a lesser chance to succeed than a proficient character.

Three modifier examples

My Ranger(Hal O'Jinya, lvl 5); Criminal background; proficient with Thieves Tools and Deception.

  1. Hal's dexterity is 18 (mod +4 to thieves tools). Level 5 Proficiency = +3. He can succeed on a Thieve's Tools based attempt with a DC of 27 if I roll a 20. If our cleric first casts guidance before the attempt, Hal succeeds on a DC 31 attempt if the cleric rolls a 4 and I roll a 20.
  2. Hal does not have proficiency in History. His Intelligence is 10 (+0). He can succeed in a DC 20 History check on a roll of 20, or up to a DC 24 History check if our cleric casts guidance and rolls a 4.
  3. Hal's Charisma is 8(-1), but he is proficient in Deception. He can succeed on a DC 22 Deception check with a roll of 20. If the Cleric's guidance roll hits 4, can succeed on a DC 26 Deception check with a roll of 20.

To make an ability check, roll a d20 and add the relevant ability modifier. As with other d20 rolls, apply bonuses and penalties, and compare the total to the DC. If the total equals or exceeds the DC, the ability check is a success the creature overcomes the challenge at hand. Otherwise, it’s a failure, which means the character or monster makes no progress toward the objective or makes progress combined with a setback determined by the DM. (Chapter 7, Using Ability Scores, PHB p. 174)

The only natural success on a natural 20 is on a to-hit roll: an attack roll. All natural 20 attack rolls are critical hits.

Rolling 1 or 20

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target’s AC. This is called a critical hit, which is explained later in this chapter. If the d20 roll for an attack is a 1, the attack misses regardless of any modifiers or the target’s AC. (Chapter 9, Combat, Basic Rules, p. 76)

There is no critical success/failure when Using Ability Scores

There is some text in the DMG for "critical success" or "critical failure."

Rolling a 20 or a 1 on an ability check or a saving throw doesn't normally have any special effect. However, you can choose to take such an exceptional roll into account when adjudicating the outcome. It's up to you to determine how this manifests in the game. (DMG, p. 242)

Note: saving throws also don't have critical successes, nor critical failures. But there is one exception: the unique rule for what happens on a 1 or 20 when rolling a death saving throw. While that is straying from your topic a bit, your comment under the accepted answer suggests that covering all three basic uses of the d20 would help you. The related question about rolling a natural 1 or 20 on a saving throws is here.

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    \$\begingroup\$ Hal O'Jinya I bet he's a bit salty. Bromine, chlorine or iodine...? \$\endgroup\$ – Graham Jun 18 at 13:05
  • \$\begingroup\$ @Graham His background is Criminal/Spy, not Sailor. It would be fitting Sailor Hal to be salty ... (our bard has a Sailor background ...) \$\endgroup\$ – KorvinStarmast Jun 19 at 15:08
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    \$\begingroup\$ So not a play on the word "halogen" then? Missed opportunity there. ;) \$\endgroup\$ – Graham Jun 19 at 15:29
  • \$\begingroup\$ @Graham the DM made the same point about a minute after seeing the character name. We talked a bit on how he might have gas or acid (Chlorine and flourrine make some nasty acids) as a favored weapon, but so far he has not manifested such a preference. \$\endgroup\$ – KorvinStarmast Jun 19 at 15:31
  • \$\begingroup\$ See if you can get FOOF into your game somewhere then. (The "Things I won't work with" link for that is by turns hilarious and terrifying.) \$\endgroup\$ – Graham Jun 19 at 16:04
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You take the roll and add the modifier as you normally would. There is no auto success or failure.

PHB: 174

To make an ability check, roll a d20 and add the relevant ability modifier. As with other d20 rolls, apply bonuses and penalties, and compare the total to the DC. If the total equals or exceeds the DC, the ability check is a success the creature overcomes the challenge at hand. Otherwise, it’s a failure, which means the character or monster makes no progress toward the objective or makes progress combined with a setback determined by the DM.

At no point does it mention critical or natural success. So to make a DC 25 check you will need a minimum of +5 to pass on the highest possible roll.

Critical Hits/Failures apply to attack rolls only.

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There are 3 core d20 rolls in D&D 5e.

Ability checks, attack rolls, and saving throws are the three main kinds of d20 rolls, forming the core of the rules of the game. (Basic Rules, p. 5)

  1. The first is an attack roll. On an attack roll, 1 misses and 20 hits, regardless of the target number (usually AC) required.

    • The "natural 20" rule is "natural 20 is a critical hit" -- the auto-hit is caused by it being an automatic critical hit, and critical hits are always hits; Champion Fighters can get critical hits on a natural 19, which means they also auto-hit on a natural 19 (regardless of their ATK bonus or the enemy AC).
  2. The second is a saving throw. Most saving throws have no special rules for natural 20s; they succeed or fail based on the target and your modifier. The exception is death saving throws, where 1 is a double-fail and 20 is a wake-up.

  3. The third is an ability check. Skill checks, initiative rolls, counterspell rolls -- these are all ability checks. Ability checks have no special rules for what happens when you roll a 20 or a 1 on the d20; just add your modifier and see if it passes the DC. (The Bard ability Jack of All Trades applies to all ability checks).

Every d20 a player rolls will be one of these three categories. Only attack rolls and death saves have anything special happening on 1s and 20s. (Barring other rules that interact with 1s, like Halfling Luck, or a monster who uses d20 for damage, or a magic item table that uses d20s).

House Rules

It is a common house-rule that natural 20s and natural 1s on saves/ability checks also have some extra awesome attached to them. Automatic success/failure is one example.

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  • \$\begingroup\$ I reorganized the answer and added the ref that you are drawing from. (I like how you set this answer up; clear, and certainly more concise than my answer). +1 \$\endgroup\$ – KorvinStarmast Jun 18 at 16:02
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Don't roll if it's not possible

If your modifier isn't high enough to beat the DC, even on a 20, you don't roll. If your PC's modifier is +3, and the DC is 25 then you shouldn't have them roll at all. There's no chance of success.

Thinking you have to roll for everything is one of my pet peeves w/ D&D. If there's no chance of failure you don't need to roll either. Want to pet the dog? You pet the dog. Want to melt the bars of the cage you're jailed in just by staring at them really hard? Unless you have an eye-beam based ability you stare really hard and nothing happens.

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  • \$\begingroup\$ Generally you shouldn't let them roll if there's no chance of success, however if you can set a DC as low as 25 that means it is possible, just somewhat beyond the abilities of that individual. There's nothing wrong with asking for a roll in that situation. \$\endgroup\$ – Cubic Jun 20 at 17:22
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    \$\begingroup\$ @cubic except that it wastes time, AND sets up the expectation of the possibility of success. Just imagine how pissed you'd be if your DM told you to roll, you used your inspiration to gain advantage, or luck to reroll, or some other finite resource, and then you rolled a 20 and still failed. If you can't succeed, you don't roll. \$\endgroup\$ – aslum Jun 20 at 20:43

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