I have polearm mastery and a reach weapon. I am walking towards an enemy.
Polearm mastery says that I get an opportunity attack when a creature "enters my reach." It does not specify that said creature has to enter my reach using its own movement, to my knowledge. Is "enters your reach" defined relative to you or defined relative to the playing field? Is a stationary creature capable of entering one's reach? For the "specific beats general" purposes of polearm mastery, must a creature use its movement to trigger an OA?
Best Answer
From the section on opportunity attacks:
"Creature moves", "provoking creature", and "interrupts the provoking creature's movement", all support the position that the intent of opportunity attacks is to react to another creature's movement, not your own. This is further clarified in the subsequent paragraph to mean "voluntary movement".
If the intent of Polearm Master was to subvert the general rule with a specific one, it would be clearly stated, as in "This is an exception to the general rule on opportunity attacks, in that it does not require movement on the part of the target". The phrases "enters your reach" and "moves into your reach" are--barring any explicit wording to the contrary, pretty clearly synonymous. A relative movement interpretation does not match anything else in the book, unless it is clearly described as such.
In short, "Enter" is an active verb, and had Polearm Mastery" been intended to break of the general rule, it would certainly have been pointed out by the designers in errata, interviews, tweets or Sage Advice.
In the absence of anything like that, the general rule interpretation should apply.
To solidly support this, user Korvin Starmast has kindly supplied definitive clarification:
Sage Advice Compendium, page 8: