This paper proposes and explains the design of a prototype learning tool named the DiaCog. The DiaCog visualizes dialog interactions within an online dialog game by using dynamically created cognitive maps. As a purposefully designed tool for enhancing learning effectiveness the DiaCog might be applicable to dialogs at discussion boards within a variety of online learning platforms. The most prominent instructional use of this tool refers to promoting collaborative knowledge building and critical thinking skills. Finally, this paper provides a new perceptive on utilizing visual elements and mind-maps into dialog games' interaction which may take place during the act of online learning.