This is called the Hamon Sheet.
The Hamon SheetHamon literally translates as “blade pattern” and refers to the visual marking the blade that was created during its smithing. The look and pattern of this outline depends on the hardening process the metal undergoes as clay is applied to it prior to cooling. The term seemed appropriate to name the sheet that tracks the relationship between two Samurai as their friendship cools and their conflict sharpens to a razor edge.
Here's the link to the JPG file in case you want to start using it in your A SINGLE MOMENT games!