Area Effect Spells – Cover
If a creature who is not in the area of effect of a spell (or effect) is in melee combat with another creature who is in the area of effect, it provides 1/2 cover (+2 Save) to Area Effect spells which have Dexterity saves, to the creature(s) it is in melee with. If the saving throw is made because of that bonus, the creature who provided it cover must make a save (with the same +2 cover bonus) or be affected by the spell (or effect). If both creatures are in the area of effect no special rules apply.
On a roll of a natural (1) on a check, attack or saving throw, you can choose to fumble. You may drop your weapon, or arcane focus/material component bag, slip and fall prone or similar problem. This grants Inspiration.
If you are reduced to 0 HP and not killed outright, on any turn before rolling your 3rd failure, you may choose to take a level of exhaustion, and accept a roll on the lingering injuries chart (DMG272) and gain 1hp. You may use this option once per long rest.
Swarms do not draw or get opportunity attacks.
“In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the DM might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack roll before you are seen” PHB
Interpretation – Moving out of cover makes you lose your hidden condition, unless some other factor obtains. Some actions, such a throwing a stone away from you, might serve as a distraction, at least the first time. Concealment that affects your target but not you (ex. due to different vision, or hiding in the shrubs), would permit you to attack once before losing your hidden condition. (Attacks always remove hidden). Out of combat, this is easier.