Got a few good questions from Stan and thought it would be best to answer them all here. I also included the final question in order to clarify how Stealth works in 5E, so that you all have a solid explanation. 1. Can you use Wisdom (Insight) check to predict an NPC's next move if you are familiar with said NPC, having met and interacted with them before? The answer is yes, based on the skill's description in the 5E PHB (p. 178). I would rule that, the higher you roll on the Insight check, the more accurate your prediction will be considering what you already know about the person and the situation. But remember: this is not a crystal ball. Even on a roll of natural 20, you won't know for sure what will happen in the future with this skill. 2. What is the criteria for darkness? This comes right out of the PHB (p. 183). There are three categories of light: bright light , dim light , and total darkness . The descriptions below apply to creatures with normal vision. Bright Light = day light from the sun (whether through clouds or open skies) or light from a source of illumination (torches, lanterns, and fires). Dim Light = shadows, a boundary between a source of bright light (like a torch) and the surrounding darkness, or the soft light of twilight, moon light, and dawn. Dim light creates a lightly obscured area. Creatures in dim light have Disadvantage on Perception checks that rely on sight and suffer a -5 penalty to their passive Perception scores. Total Darkness = night time outdoors without moon light, an unlit dungeon or subterranean area, or an area of magical darkness. Total darkness creates a heavily obscured area. Creatures in total darkness effectively suffer from the blinded condition. 3. How does darkvision work? First, darkvision allows you to see in dim light as if it were bright light instead. This means you can see normally in dim light without penalty, at least within the limits of your darkvision (usually 60 feet). Second, darkvision allows you to see in total darkness as if it were dim light instead. In other words, you do no suffer the blinded condition while in total darkness, and you can see up to the limits of your darkvision (usually 60 feet)... BUT you suffer Disadvantage on Perception checks and a -5 penalty on your passive Perception score, meaning its very hard to find secret doors or hidden traps, for example. Furthermore, you can't discern color in total darkness, only shades of gray. 4. How does the Gloomstalker Ranger's Umbral Sight ability work? Well, the ability reads: "You are also adept at evading creatures that rely on darkvision. While in [total] darkness, you are invisible to any creature that relies on darkvision to see you in that darkness" (Xanathar p. 42). So, here are a few examples to show how this ability works: Example #1 : The gloomstalker ranger is in total darkness. A regular human with normal vision walks nearby in a dimly or brightly lit area. The human is effectively blinded from seeing the gloomstalker ranger in this case. Example #2 : The gloomstalker ranger is in total darkness. An orc with darkvision up to 60 feet walks by. Normally, the orc would see into the total darkness that the ranger is standing in and treat it as dim light, meaning the orc would automatically see the ranger if the ranger wasn't hidden, and if the ranger was hidden, the orc would get a Perception check to notice him (but at disadvantage, because the area is treated as dim light). However, because the orc can only see the ranger due to his darkvision ability, that means the orc treats the ranger (who might be standing there openly or might be hiding) as invisible. Even if the ranger fired an arrow at the orc and wasn't hiding at all, the orc would not be able to see the ranger. The orc would only know that an arrow came out of the area of total darkness and approximately which direction it fired out from. To combat this, the orc would need to light a torch or some other source of illumination, which would turn the area of total darkness into an area of dim light or bright light (depending on how close the orc got with the torch). At that point, the ranger would automatically be spotted if he wasn't hiding, and if he was hiding, the orc would get a normal Perception check to spot him. Example #3 : The gloomstalker ranger is hiding in dim light. An orc passes by. The orc's darkvision works normally in this situation because Umbral Sight only activates if the ranger is in total darkness. In other words, the orc would get a Perception check to spot the ranger in the area of dim light without penalty, because the orc's darkvision allows him to see in dim light as if it were bright light. And in bright light, the orc does not suffer any penalty on his spot check. Example #4 : The gloomstalker ranger is outdoors in the woods. It is a full moon or a night full of starry twilight. The outdoors area is treated as dim light. Therefore, his Umbral Sight doesn't usually work. However, there might be patches of total darkness, in delves hidden by heavy foliage, or other obvious situations (on a case-by-case basis; the player should ask the DM if he can see any patches of total darkness on the map). Example #5 : The gloomstalker ranger is outdoors in the woods. It is a cloudy night or a new moon. The outdoors area is treated as total darkness. Therefore, the ranger's Umbral Sight works, allowing the ranger to remain invisible against creatures using darkvision to spot him. However, any amount of torch light or another source of illumination would end this effect, so long as the area the ranger is in lies within that light source's radius. So, to recap: this ability only works if the ranger is within total darkness. If he isn't in total darkness, it doesn't activate. 5. How does Stealth work? There are several factors which affect your ability to use the Stealth skill to hide from enemies: illumination , obscurement , and awareness . When you are in bright light and you try hiding from (or sneaking past) a creature that is looking in your direction or can already see you , then the creature automatically spots you without needing to make a check. When you are in bright light and you try hiding from (or sneaking past) a creature that is not looking in your direction all the time or that might turn away to look in another direction at random intervals, then you get to make a normal Stealth check opposed by the enemy's Perception check. This check represents your chance at sneaking past the creature when it isn't looking at you; if you fail, the creature turns and spots you while you're in the open, or you possibly make a sudden noise which draws its attention and causes it to turn and see you. When you are in dim light or light obscurement (patchy fog, dungeon furniture, underwater, or moderate foliage) and you try hiding from (or sneaking past) a creature that is looking in your direction or can already see you , then the creature automatically spots you without needing to make a check. The one exception to this rule is that Wood Elves have a racial ability which allows them to hide in these conditions. When you are in dim light or light obscurement (patchy fog, dungeon furniture, underwater, or moderate foliage) and you try hiding from (or sneaking past) a creature that is not looking in your direction all the time or that might turn away to look in another direction at random intervals, then you get to make a normal Stealth check opposed by the enemy's Perception check, which suffers Disadvantage due to the dim light conditions; if passive Perception is used, then the enemy suffers a -5 penalty to it. When you are in total darkness or heavy obscurement (opaque fog, opaque water, or dense foliage) you automatically avoid detection due to line of sight. Creatures that might normally "see" you within range instead are treated as if they suffer from the blinded condition when trying to notice you. However, if you are moving around at all in the darkness and there is a chance the creature might hear you, you get to make a normal Stealth check opposed by the enemy's Perception check, which suffers Disadvantage. If the creature succeeds, your movement makes a noise, which raises the creature's awareness that it is not alone. The creature knows the rough direction of the sound, too, but it still can't see you hiding in the darkness. If you try to surprise a creature that already knows you are there with a sudden attack or other similar action, you must first make a DC 20 Wisdom (Insight) check to successfully determine the most opportune time to attack the target. Then you would make a Stealth check opposed by the target creature's Perception check; if your Insight check failed, then your Stealth check suffers Disadvantage. If you are in the middle of combat and an enemy has already spotted you before, and you try to hide from it, you must first break its line of sight to you by moving out of sight entirely. But even if you manage to do this, the enemy knows you are there. As a result, it gains Advantage on any Perception check to find you while you are attempting to hide from it, as long as you remain in the same broad area. In order to remove this Advantage effect, you would need to leave the immediate area and hide somewhere else; at that point, the creature does not gain Advantage anymore on its Perception checks to detect you, even if the creature follows after you in the same general direction that you fled.