Block ◇
Trigger: You are wielding a melee weapon and an opponent you can see attempts to move into a square adjacent to you.1
You move into the square first, before the opponent can. The opponent may revise their movement to move into a different square instead. This new square must be adjacent to both their current square and the square you just blocked, but it cannot be the square you just vacated. Additionally, if you moved diagonally to block the square, this revised movement also cannot cut across the diagonal line between your starting square and the new square. If the opponent chooses not to revise their movement, they waste the 5 feet of their movement speed they spent trying to move into the blocked square.
There is no limit on how many times you can use this action.
There are several edge case limitations and usage notes for this action:
1. If the opponent started their move adjacent to you and is moving non-diagonally into a space diagonal to you, you cannot block their movement. You may instead use this action to move 1 square in the same direction as them, provided that you can legally occupy the target square.
2. You must be able to move to use this action, and the target square must be one you could legally move into. For example, you cannot use this action when restrained or immobilized. You also cannot move through obstacles, such as a barred window.
3. You cannot use this ability if normally moving into the target square would cost more than 10 feet of movement. This means, for example, that you cannot move diagonally to block in rough terrain.
4. This movement is itself movement and may trigger actions such as the Block action by another character. Any characters whose action triggered a Block cannot take any other actions or reactions until the original movement is resolved. If multiple characters attempt to Block the same square, all Blocking characters make a melee weapon attack roll. The highest attack roll gets to occupy the square (ties go to the blocker who used the action earlier in the chain). The most common scenario where this multiple blocking case comes up is when one of the original mover’s allies attempts to screen you in order to create an route for the original mover to get around you.
(Design notes: the purpose of this action is to allow melee characters to hold the line and prevent opponents from getting past them.)