Can an enemy facing someone with the Sentinel Feat and a Glaive move 5' then disengage?

Question

Sentinel (pages 169–170, Player's Handbook) says

• Creatures within 5 feet of you provoke opportunity attacks from you even if they take the Disengage action before leaving your reach.

So one of my characters has the Sentinel feat and uses a glaive. The GM moved the bad guy back 5' so he's no longer standing within 5' of my character, then used the disengage action to avoid an OA, and then completed his original move action. Is this valid by RAW and/or RAI?

I know Rule 0 "the GM can rule however he wants".

Answer 1

The creature will still provoke an opportunity attack when it leaves your range.

The original sentinel feat states the following:

Creatures within 5 feet of you provoke opportunity attacks from you even if they take the Disengage action before leaving your reach.

It is clarified by the following official errata from WotC

Sentinel (p. 169). Ignore “within 5 feet of you” in the second benefit.'

Therefore, the OA attack works for you as soon as the creature leaves the range of your melee attack, in this case, 10 ft.

Source: https://media.wizards.com/2015/downloads/dnd/Errata_PH.pdf

Answer 2

Looks legit to me. When the creature left your reach it was not within 5 feet of you so the second part of sentinel is N/A.

Answer 3

The feat gives you the Attack of Opportunity at 5'.

The feat states:

Creatures within 5 feet of you provoke opportunity attacks from you even if they take the Disengage action before leaving your reach.

Which means as soon as a creature who is within 5 feet of you attempts to move away, you are able to take an opportunity attack regardless of whether or not they have disengaged. That you are using a reach weapon is irrelevant as the specific wording of the feat permits you to make an Attack of Opportunity at 5' anyways. So even if the creature stepped back 5' just to try to disengage and run away, it's already too late. The initial 5' step already triggered the feat and provoked the attack.

If you'll take note, the feat does not say the OA happens when they leave your threatened area. It says "creatures within 5 feet of you provoke opportunity attacks".