Thank you! I'll have to add come key customization options. The A/S/C scheme is comfortable for me, but maybe a more traditional default scheme might be A/S/D or Z/X/C?
The blood effect isn't really complex, although this explanation might get a little technical. When you damage an enemy, it spawns some master particles that have a velocity corresponding to the direction the attack came from. These particles can bounce off the terrain a little, and they become more transparent over time. Each frame, each of those master particles spawns a blood particle at a random location within 4 pixels of the master particle. Blood particles inherit the master particle's color and transparency, but not its velocity. Instead, they simply fall downwards. Unlike master particles, when blood particles collide with terrain they stop moving. Additionally, the collision does not cause them to be pushed out of the terrain, allowing for a more spread-out blood effect as opposed to a flat effect sitting on top of the ground.
Hope that answers your question!