Stealth/Hiding Mechanics

Hiding in combat is broken into a simple flow of requirements, actions, and then the result.

1) Requirement: "Unseen"
2) Action: "Hide Action"
3) Result: "Hidden Condition"
4) Requirements: "Maintaining Hidden"

Being Unseen

To begin hiding, whether it be in or out of combat, you must be unseen by the creature you are attempting to hide from. You can meet the unseen prerequisite by being Heavily Obscured or behind Three-Quarters Cover or Total Cover. If you are not behind full cover, also must not be within the creature's line of sight. if you can see a creature, you can discern whether it can see you.

If there are multiple creatures, they may use their free action to alert a creature that is being "hidden from" allowing them to take the search action as a reaction.

Some features or traits such as Wood Elf's "Mask of the Wild" expand the ways you can be "unseen". Magical effects such as the Invisibility spell make you "unseen" as well, but other countermeasures such as see invisibility remove the effects ability for hiding.

The Hide action

Once you have met the prerequisite of being unseen, you may attempt the hide action. Some features such as a Rogue's cunning action allow you to do so as a bonus action. When you take the "Hide" action, roll a Dexterity (Stealth) check the DC of which is equal to the Passive perception of the creature you are hiding from. Certain factors such as creature alertness, and lack of "cover" or other environmental factors that would provide the creature with advantage, instead add a +5 to the passive perception, as well as the later bonuses provided by stacking advantage (Eg, 1 = +5, 2 = +5+2, 3 = +5 +2 +1, Ect)

In some scenarios, the DM may determine that hiding is impossible due to terrain or enemy factors. While you may be able to hide from an ogre in the heat of battle due to its back being turned, a multiheaded creature such as a hydra may prevent this.

The Hidden Condition

Upon meeting the prerequisite and beating the passive perception DCs needed to gain the "Hidden" condition, you are under the following benefits:
- Attacks you make gain advantage
- Creatures you are hidden from cannot target you with any abilities that require "sight" or "a creature you can see"

The hidden condition is representative of not being seen by your enemy, and a general lack of knowledge to where you are. However, the stealth check itself is more representative of how you avoid giving away your position while unseen. The other senses are what you are trying to fool, creatures that rely more on Hearing or Smell may be harder to hide from. Creatures with special senses such as tremorsense or blind sense may require additional or more difficult prerequisites to hide from, or flat out might not be possible to elude.

Maintaining the Hidden condition

There are a few key "actions" or events that may reveal you, removing the hidden condition and its benefits;
- Ending your turn within the line of sight of the creature you are hidden from
- Making a noise that can draw the attention of the Creature
- Making an attack roll, or casting a spell with a verbal component
- Failing an opposed stealth check against an enemies search action & resulting Wisdom (Perception) Check

Footnote: Invisibility
When you have the invisible condition, you have the unseen condition unless the creature you are hiding from "sees" in a manner that does not rely on seeing objects on its current plane.

The Invisible condition provides an additional benefit:
- Surprise. If you're Invisible when you roll Initiative, you have Advantage on the roll.

Footnote: Passive Perception
Passive Perception is a score that reflects a creature's general awareness of its surroundings. The DM uses this score when determining whether a creature notices something without consciously making a Wisdom (Perception) check.

A creature's Passive Perception equals 10 plus the creature's Wisdom (Perception) check bonus. If the creature has Advantage on such checks, increase the score by 5. If the creature has Disadvantage on them, decrease the score by 5. For example, a level 1 character with a Wisdom of 15 and proficiency in Perception has a Passive Perception of 14 (10+2+2). If that character has Advantage on Wisdom (Perception) checks, the score becomes 19.

Though if players are doing something that's easy to be found or heard, like sneaking up to a foe's back in a quiet place, DMs could give enemies Advantages in Perception, thus make their Passive Perception much higher by ganting a +5, by RAW. DMs also still can decide whether is the current situation appropriate for players to take the Hide.