ISO app/plugin for tags

Hi!

I'm looking for an app/plugin option that will automatically send a notification or direct message to a user when they tag another (specific) user. 

This is how I would like it to work.

1. Lucy is writing a post and tags "@John".
2. Lucy automatically gets a direct message or notification alert with a message.
   1. Message/notification would either
      1. Contain a video & text embedded within OR
      2. Contain text + a hyperlink to a specific page (where we can embed a video).

Does anyone know if there's an app/plugin that can do this? Or is this something we would have to get a custom developed?

Thanks!