Is it possible to ready a ranged weapon attack (specifically with a longbow) when you suspect there might be an enemy lurking and hidden, completely concealed from you?
For example, my party and I are walking a long, narrow canyon with many ledges and nooks and crannies and we get ambushed by several Orcs with melee weapons from the ground.
Following initiative and the start of the battle, my turn comes up and I have no clear line of sight on them as it is too narrow to have a clear shot with my bow with several party members partially concealing my foes.
I would tell my DM: "I use my action to ready a ranged attack against any potentially concealed enemy that shows up from above to range attack us"
Is that a legal action and do I have to declare a line of sight? Or do we assume my character is looking all around to find this potentially hidden foe?
Best Answer
Absolutely
Once initiative has been rolled and your team is in combat, there's no longer a possibility of surprise. At that point, those hidden enemies likely are following their own initiative rolls, and in a sense, are participating in the same combat you are, if only in the sense that you can't see them and they're choosing not to attack.
In 5e, it is assumed that you are paying attention to all angles around you, so as soon as a new enemy shows up, you can definitely shoot at them with your held action.
Keep in mind that since you are focusing on attacking as quickly as possible, by spending your action to "be ready" for an ambush, your DM may not allow you to use a proper Perception check since your action is already expended, so you may not notice as soon as your party members do.
A standard trigger for a held action would be "If an enemy walks towards me, I attack it", which could easily convert to "If I notice a new enemy" or "If my allies point out a new threat".