I was chatting with one of my party mates, and we were debating whether the Sentinel feat allowed me to get an opportunity attack if the enemy is approaching me. The reason I ask this is because I wanted to use a lance, seeing as how the lance gives me a reach of 10 feet.
Because nothing in the wording of the feat says specifically that it has to be an enemy leaving, do I still get that opportunity attack?
Best Answer
Not if you only have the Sentinel feat
The Sentinel feat (PHB, p. 169-170) is specific on when it engages:
and
In the case you provide of a creature entering your reach, neither of those qualifiers are met. The creature isn't leaving your reach, it is only just entering and the creature isn't in 5' of you and hasn't attacked an ally. Therefore you don't get an opportunity attack upon the creature entering your reach.
The Polearm Master feat does synergize with this, but not with a Lance
The Polearm Master feat (PHB, p. 168) says:
This is the feat/clause that let's you take an OA when a creature enters your reach. Without it, you can't do it :(
But there can be only one
It is important to remember that you only get one reaction that refreshes at the start of your next turn.
This means that you can either use that reaction and take an opportunity attack when a creature enters your reach (via Polearm Master) or use it when leave your reach (via Sentinel). You can't use both, so you'll have to decide if it is better to take it when the creature is coming or going.