The CyanogenMod team has announced the availability of CyanogenMod 12.1 nightlies for HTC One M9 through their Google Plus account. The first nightly release of CyanogenMod 12.1, based on Android 5.1, was announced on April 16, 2015. A stable snapshot build was released on September 1, 2015 but nightly builds continue to roll out every day to reach some devices. Beside they released the nightly version for HTC One M9 with its codename: himaul, the other nightly version is available for the Nvidia Shield Tablet, Shield Portable, and the Lenovo ZUK Z1 on this week. These 3 nightly versions have different and unique codenames: shieldtablet, roth, and himaul, respectively.
Source: htc.com
The file CyanogenMod 12.1 nightlies Build can be downloaded on their official website. This nightly version is the experimental version and still in developing process. If you have an HTC One M9 or any device that was mentioned above, and you are interested to change the OS to CyanogenMod, you have to be ready for some risks. The installation could lead to a bricked device, and worse, could also void your handset’s warranty.