My DM has introduced a house rule affecting opportunity attacks, and I am trying to understand the implications of this change.
The new rule is this:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach or moves inside your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.
I've bolded the portion added by the house-rule.
To clarify, when a creature moves from out of a character's reach into their reach, it does not provoke an opportunity attack. But if a creature moves within a character's reach (i.e. from one spot within reach to another), it provokes an opportunity attack.
I think this can reduce the utility of the Rogue (with Sneak Attack), and kind of reduce the Mobile feat's benefits. Are there other places where this rule can change how combat is played? Like, what can become imbalanced with this ruling?
My goal is to better understand this mechanic before talking with my DM.