This is an Arduino library for controlling external computer monitors over the VESA DDC/CI standard. This standard is present on many different types of connectors but depends heavily on the actual manufacturer to implement it properly. Check if your setup is actually compatible with this.
Some simple examples can be found in the examples directory.
More Functionality can easily be added. See ddcutil for some further reference on VPC codes.
The Arduino is to be connected to VGA, HDMI or DVI using the corresponding pins for DDC. This is essentially an I2C connection and uses the standard arduino Wire library so use the appropriate pins for I2C on your microcontroller.
Disclaimer: I only tested this with a VGA connection, however DVI and HDMI should work as well. Pay attention to using the correct voltages though!