Let's say that a player uses the ready action and says "If a character aims at me with a weapon, I'll use Teleport to move behind cover."
What happens? Does this work? If not, please cite the rules to explain why not.
[RPG] Can you use a readied action to teleport out of the way of a weapon attack
dnd-5ereadied-actionteleportation
Best Answer
Yes, you can avoid the attack with that wording.
PHB p.193 explains that the Ready allows you to decide what perceivable circumstance will trigger your reaction. Your reaction happens right after the trigger finishes.
Your stated trigger is "If a character aims at me with a weapon, I'll use Teleport to move behind cover."
The act of aiming at you with a weapon is a "perceivable circumstance", which occurs before the act of firing a weapon happens. It thus triggers the reaction before the attack has a chance to resolve. There is no requirement for the attack to occur, because you did not state your trigger to be "when I am attacked".
Giving up your Action to prepare a Readied Action is a risk, and has the reward of giving you finer control over your activity in combat at the loss of some amount of agency. If the trigger never occurs, you lose any action at all, for instance. You also lose multiple attacks if you have them.
Another risk of stating a Readied action that finely is that the anticipated action might never occur. For instance, an enemy could be raising his weapon to cast it aside or to signal his folowers to stand down. Stating "when a spellcaster starts casting" as a trigger could cause you to use it when the spell caster is about to raise his arms in surrender or to ask for a parlay, if the action looks enough like spellcasting, Some DM interpretation is warranted.
To support this view, here is a relevant tweet from Lead Designer Jeremy Crawford (emphasis mine):