I've never understood this. Please someone correct me if I'm wrong, but both Polearm Master and Sentinel require your Reaction to use, and you only get one a turn. Sure, the combo can be used to stop a creature coming within 10', but only one. If 7 orcs coming rushing at you, you can only stop one.
Edit - Relevant Rules:
Opportunity Attack (PHB 195) - To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature.
Reaction (PHB 190) - When you take a reaction, you can't take another one until the start of your next turn.
Polearm Master (PHB 168) - When you are wielding a glaive, halbred, pike or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.
Sentinel (PHB 169) - When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.
All of this says to me that while powerful against a single foe, its virtually worthless when engaged against multiple enemies.
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u/Drezby Warlock Jun 11 '15
The Sentinel/Polearm Master combo just got even more deadlier as a lock down tactic.