Do the Tunnel Fighter UA fighting style and the Polearm Master feat combine to grant indefinite opportunity attacks?

Question

Here's the scenario:

The character in question has both the Tunnel Fighter fighting style from Unearthed Arcana: Light, Dark, Underdark!, which says:

You excel at defending narrow passages, doorways, and other tight spaces. As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5ft while within your reach.

As well as the Polearm Master feat (PHB, p. 168), the second bullet point of which says:

While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.

Assuming you hit every target that enters your reach (10 feet, unless you're using a quarterstaff), could you essentially take out a stampede of kobolds who have no ranged weapons? (Or some other instance of lots of easy-to-hit targets rushing at the player.)

Am I correct in assuming this means all approaching targets are attacked once as they enter my reach? Am I missing something that says otherwise in this scenario?

This combination seems really good to protect the rest of the group from a swarm of enemies and to abuse choke points. It's almost too good against weaker enemies.

Answer 1

You are correct.

Tunnel Fighter lets you make opportunity attacks for free, and Polearm Master lets you do so at a distance.

An intelligent or reasonably aware force would have a good chance to figure this out after you skewer the first few.

As for "too good," it's powerful but not unrealistic. Choke points are good strategy in the real world, too; see the Battle of Thermopylae or various castle designs that involve narrow gates or low doorways. Given that the PC has spent a precious feat and their fighting style choice on this, they deserve to have it be pretty useful.

Answer 2

You are reading the combination correctly. A Tunnel Fighter with Polearm Master can produce a formidable blockage. The opportunity attacks you're allowed to make against those entering your reach are not limited by the action-economy.

It is really good, in a very specific situation. You need to be able to avoid their ranged attacks and any areas of effect, and those whom you don't eliminate upon entering your reach can still bull-rush you in numbers to grapple you or knock you down.

Answer 3

Yes you can do this in theory; No you can't do it to kobolds.

It says what it means and it means what it says. You use your bonus action to take up the defensive stance. A creature you can see entering your reach without taking the Disengage action is subject to an opportunity attack due to your Polearm Master feat; this does not use your reaction due to your Tunnel Fighter feat. Therefore you can do it again and again.

You can't do this to kobolds because: Tucker's Kobolds. Kobolds are not that stupid.

It is really good to protect the rest of the group from a swarm of enemies because that is exactly the situation it is designed to be good for. It is less useful against, say, a Red Dragon's breath weapon: when that happens the feats Resilient (Dexterity) and Shield Master are much better.

Answer 4

You're exactly right. I'd imagine this combined with the sentinel feat would be near unstoppable against a horde of enemies. However not only does this cost you TWO feats and a fighting style but the situation is so specific that is will probably never occur unless you specifically ask to DM to include it.