culmqtt is a small connector between a CUL (in my case the CC1101) and a MQTT server. It was written as I was migrating my home automation from FHEM to Home-Assistent which lacks support for that CUL. Sadly some of my devices (for example the FS20 door-window-contacts) did not have an equivalent supported by Home-Assistent.
At first there was a shell command to forward Home-Assistent commands to FHEM which would execute them through the CUL. In order to shut down FHEM completely culmqtt replaced FHEM as the bridge between Home-Assistent and the CUL.