What I mean is if a reaction has a trigger that doesn't involve an attack against you, do you actually have to be aware of it to use your reaction or as long as the reaction meets the trigger requirements you can use it?
For example
Champion Glimpse of redemption if your ally and the enemy are both within 15 feet of you but on the other side of a pillar or something where there is no line of sight or effect and you can't see them in any way, could you still use Glimpse of redemption as a reaction to an attack made?
