The Sentinel feat allows you to make opportunity attacks in response to enemy movement. If they target any ally or leave your melee reach, you can attack them as they leave. You will automatically leap towards the commander provided they have moved out of range and not attacked anyone else. You may also take a reaction after your turn is over or at any time that an enemy steps away from you for more than half their movement speed if possible. The only times when this won’t work are after the first round of combat, during battle strategies or before initiative rolls are made when either player declares what actions they’ll take on their turn in advance; such an anticipation removes any need to invoke it afterwards since decision making has already been covered at that point.
For instance, let’s say an enemy is standing right next to your party healer and you’re the only one who can reach them:
– If they turn their back to you and walk away, you may take a reaction between their movement speed and make an opportunity attack as long as they leave melee range.
– If they move up next to you and attack, no reaction is available since they’re already in range.
– If they use another movement speed that leaves them out of melee range, such as a dash or disengage, you may take a reaction before the end of their turn and make an opportunity attack if they leave your reach.
– If they use another full movement speed that is not an action, such as a dodge or primal surge, you may take the reaction at any point they do this if possible.
– If they cast spells or use ranged attacks, no opportunity attack is available since they’re outside your melee reach.
– If they use a ranged spell or ability on an ally, you may take the reaction after their movement is finished at any point after it has been determined that no opportunity attack was available.
In this way, the Sentinel feat works very much like the flanking rules for melee characters. You can never be flanked by a square or hex your character occupies, so it’s generally preferred to have the enemies only move half their movement speed instead of taking advantage of any possible movement rate past that.
This also means you cannot use this feat if the enemy is using an extra action to run away. Using a second turn would still give them enough time to counterattack before leaving your reach, whereas using another movement speed would not.