Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it.
And in some cases it has a knife...
This mini guide is meant to be a go to place for people who wish to better understand stealth and hiding but also covers such things as Surprise, Light and Darkness within 5th edition. Before we start I will list the relevant rule sections and pages:
Chapter 7 Ability Scores (pages 175 Passive Checks + Group Checks, 177 Stealth + Hiding, 178 Perception),
Chapter 8 under Stealth, Noticing Threats, Encountering Creatures and Surprising Foes, Vision and Light + various senses on pages 182-185.
Chapter 9 Hide on page 192, Unseen attackers and targets on pages 194-195, Opportunity Attacks page 195.
Appendix A Conditions on page 291 for Invisibility.
Pages 8-9 for the various senses.
June 10 2015 - PHB errata document: http://dnd.wizards.com/articles/features/ph_errata
NOTES ABOUT ERRATA
-The June 10 2015 errata has changed the way the sight restriction for hiding works as a whole. It explicitely states that the question when it comes to sight is no longer "can an observer see me?" but instead "can an observer see me clearly?". This is a nice boon for hiding in general since it means that many new situations will allow a creature to roll to try to hide. What constitutes being "clearly seen" is left up to the DM to determine, however it is logical to expect anything that makes you uncleartly seen to work. Light obscurement, heavy obscurement, half cover, 3/4 cover would all fall into this conclusion since they all make you unclearly seen. But ETV is going to be the best recommendation this guide can make.
Part 1 Hiding:
When you want to hide, you make a Dexterity (Stealth) check. This check is against the Passive Perception (determined by each creature's Wisdom score, more on this below) of creatures around you. Several factors revolve around becoming and remaining hidden. Clear sight and noise being the most common.
What does hiding get me in general?:
-Advantage on attack rolls against creatures you are hiding from.
-If you manage to beat a creature's Passive Perception score, you are considered hidden - both unseen and unheard - from it.
-Creature's who can't see you have disadvantage on attack rolls against you. This is true weither the attacker is guessing the target's location or if it the attacker is targeting a creature it can hear but not see (an invisible one for example). If the hiding creature is not in the location that is targeted by the attack, the attack automatically misses.
-There are many more benefits possible depending on your class, race, etc... Monsters also have many unique abilities and features that player characters cannot access easily if at all.
What does being invisible get me in general?:
-An invisible creature is impossible to see without the aid of magic or a special sense (see below). For the purpose of hiding, the creature is heavily obscured. The creature's location can be detected by any noise it makes or any tracks it leaves.
-Attack rolls against the creature have disadvantage, and the creature's attack rolls have advantage.
What is the difference?:
-An invisible creature does not lose its benefits after making an attack unless whatever grants you the invisibility specifies that you do.
-If you are hidden and invisible and a creature beats your Dexterity (Stealth) check, you do not lose invisibility unless whatever grants you the invisibility specifies that you do.
Restrictions when trying to become hidden or remain hidden:
-You can't hide from a creature who can clearly see you. If you have Total Cover (such as standing behind a wall which covers you completely) or are Heavily Obscured (such as standing in complete darkness) from a creature, it cannot see you as you are completely concealed. The June 10 errata has changed the limits for sight to specifically say it's about being clearly seen and no longer simply "seen". Another way is invisibility which causes you to be unseen(see below).
-In combat, it is assumed by the edition that most creatures are alert (this is DM fiat). So if you move out of hiding (from behind a rock, tree, corner, or whatever was stopping a creature from clearly seeing you) and approach someone you don't necessarily remain hidden. If your DM determines that the creature is distracted, you may be able to remain hidden and gain advantage on an attack before you are clearly seen.
-If your Dexterity (Stealth) check doesn't beat a creature's passive perception, you are not hidden from it. This can be important if creatures enter the area where you are hiding as your stealth check is immediately compared to their passive perception.
-If you make noise (such as shouting a warning or knocking over a vase), you give away your position and lose hidden.
-if you attack while hidden, you lose hidden and give away your location when the attack hits or misses.
-If your Dexterity (Stealth) check is beaten by a creature's active perception check (using the Search action as detailed below), you are no longer hidden from it.
Ways to see/detect hiding or invisible creatures automatically:
-Blindsight allows a creature to perceive its surroundings without relying on sight, within a specific radius. This allows a creature to see invisible creatures or creatures who are hiding. Examples of creatures with this sense would be creatures without eyes, creatures who use echolocation (bats) or have heightened senses (true dragons).
-Darkvision allows a creature to see in the dark within specific radius. If a creature is using darkness to hide, it will not be able to hide from a creature with darkvision. This is a very common sense, especially on creatures who typically live underground.
-Tremorsense allows creatures to detect and pinpoint the origin of vibrations within a specific radius, provided that the creature and the source of the vibrations are in contact with the same ground or substance. Unless the creature hiding is flying or incorporeal, hiding creatures are detected by this sense.
-Truesight is the ultimate sense. It allows creatures up to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the creature can see into the Ethereal Plane within the same range.
-If a creature is using Complete Darkess to hide and a light source illuminates the square the hidden creature is in, it will automatically lose hidden since it can now be seen clearly.
-If a creature is using Total Cover to hide (such as standing behind a wall) and an observer places itself in a way that the hidden creature no longer has total cover, the hidden creature will automaticaly lose hidden since it can now be clearly seen.
Part 2 How Passive Perception works:
Passive perception is already on monster stat blocks. However to determine the passive perception of a character, you simply use the following formulae:
10+Wisdom Modifier. So if you have a 14 Wisdom on a 1st level character, your Passive Perception score is 12 (10 base + 2 from Wisdom). If you have proficiency in Perception, you gain your proficiency bonus. Your score then becomes 14 (10 base +2 from Wisdom + 2 Proficiency).
If you have a negative Wisdom Modifier, you simply deduct your score. So with 9 Wisdom, you have a -1 modifier. If we use the same character as above, your Passive Perception is now 11 (10 - 1 from Wisdom + 2 Proficiency).
When you roll a Dexterity (Stealth) check you compare it to the Passive Perception of creatures around you. So if you roll 15 Stealth and a creature has 14 Passive Perception, as long as you meet the criteria to become hidden (see Part 1) you are now hidden from that creature.
Finding hidden creatures the hard way:
You can use your action to make a Wisdom (Perception) check via the Search Action to try to find a hidden creature or creatures. The result of the check is compared to the Dexterity (Stealth) check of the hidden creature(s). If you tie or beat the Stealth check, that creature or creatures are no longer hidden, though you may not be able to see them in the case of an invisible creature for instance.
Part 3 The things that help to hide
-Invisibility stops creatures who don't have Blindsight, Tremorsense and True Sight from clearly seeing you. Which means you can make a check to hide whenever you want as long as you are invisible. However the PHB states that signs of an invisible creature's passage may still be noticed and you still have to remain quiet. (see above for what it gets you)
-Having proficiency in Stealth means that you get to add your proficiency bonus, this can be as high as your Dexterity score for example and so is very useful to have.
-The Lightfoot Halfling's Naturally Stealthy feature allows you to attempt to hide even when you are obscured only by a creature that is at least one size larger than you (Medium and above).
-Wood Elf's Mask of the Wild feature allows you to attempt to hide even when you are lightly obscured by foliage, heavy rain, falling snow, mist, and other natural phenomena.
-The Skulker feat gives you many benefits which revolve around stealth. You can try to hide when lightly obscured from the creature from which you are hiding. When you are hidden from a creature and miss it with a Ranged Weapon Attack, making the attack doesn't reveal your position. Dim light doesn't impose disadvantage on your Wisdom (Perception) checks relying on sight. However it should be noted that the Errata released on June 10 2015 has changed the way stealth works. The question is no longer "can I be seen?" but "can I be seen clearly?" which could mean that this feat is now redundant for hiding. Light obscurement is by definition something that makes you unclear for sight outside of things like creatures with Darkvision (which turns light obscurement into bright light). This is unclear at the time of this update so one should expect table variation.
-More on this section soon
Work in progress