Drop Prone (5e Variant Rule)
From D&D Wiki
This variant rule uses the idea of diving for cover when under fire. It provides all characters a defensive option against ranged attacks, though at the cost of mobility. When and where it can be used safely and strategically may be limited, as being prone grants advantage to attacks made within 5 feet, but it is still a flavorful option to add to your game, nonetheless.
When something you can see attacks you and you are aware it is happening, you can use your reaction to immediately fall prone before the attack rolls to hit but after it targets you.