Is the Tunnel Fighter stance once per enemy, or how often does it trigger?

Question

Does the Tunnel Fighter fighting style (from UA: Light, Dark, Underdark!) trigger multiple times on an enemy who is, for example, running in circles around the character?

If yes, does it trigger every 10 ft. stretch (more than 5 foot movement), or every space after the initial trigger (since each move IS over 5 feet of movement after the grand total is taken into account)?

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I realize that I misread the UA description of the Tunnel Fighter fighting style. I thought it meant unlimited attacks on 5+ movement within range but nothing to do with regular oppies which used your reaction. I now realize it is completely the opposite (unlimited regular oppies, but the 5+ movement attack uses the reaction)

Answer 1

Unlimited Opportunity Attacks, but only one Reaction for the 5' movement.

Tunnel Fighter states (emphasis mine):

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 5 feet while within your reach.

The main gist of this is that with Tunnel Figher, Opportunity Attacks no longer utilize your single Reaction and therefore you can take as many Opportunity Attacks as have been triggered.

In the case of an enemy "running circles around a character", the following would need to occur:

1. Creature enters your reach
2. Creature moves at least 5' (can expend your one reaction to make a melee attack against them). Note that this is NOT an Opportunity Attack, but simply the use of your single reaction.
3. Creature moves out of your reach and triggers an Opportunity Attack

If the creature chooses to Rinse and Repeat (but why, man, why?!), then the cycle begins for 1 and 3 and skips Step 2 (you still only get one reaction.)

Unearthed Arcana isn't necessarily balanced

Having unlimited Opportunity Attacks may be unbalanced, and may be why they never made this official. Unearthed Arcana is playtest material, and this particular UA has been out for 3 years with no updates or official release. DM beware :)