What is 5e tunnel fighter?

As an AI language model, my training data only goes until October 2021, and I do not have information about 5e tunnel fighter beyond that period. However, I can give some general information about the concept of a tunnel fighter in Dungeons and Dragons 5th edition (5e) based on my database.

In D&D 5e, a tunnel fighter is a type of combat strategy that is specifically designed for fighting in narrow or confined spaces such as tunnels, caves, and corridors. The idea is to use the environment to your advantage by setting up a defensive perimeter and funneling enemies into your kill zone.

The concept of the tunnel fighter is introduced in the optional rules section of the Player's Handbook, which includes a set of fighting styles that characters can choose from at certain levels. Tunnel fighter is one of these fighting styles that provides benefits when fighting in tight spaces.

Characters who choose the tunnel fighter fighting style gain various benefits including the ability to make opportunity attacks against creatures that move within their reach, even if they take the Disengage action or use their movement to disengage. Additionally, they can set up a defensive perimeter that imposes disadvantage on enemy attacks made against them within five feet.

Overall, the tunnel fighter fighting style can be a potent and effective strategy for characters who expect to fight in confined spaces. However, it may not be as useful in open environments where enemies have more room to maneuver.