Enabling ADB on Nexus 6 with dead screen
I followed this guide


I went into fastboot mode by holding down the power button (force restart) and the volume down key (recovery mode)
I downloaded a twrop image from: https://dl.twrp.me/shamu/
fastboot flash recovery twrp.img
fastboot reboot (while holding the volume down key see note)
Then I booted the recovery image
adb with root permissions detected my device. If you get a permission denied error try killing adb and running with sudo


*Note many devices will replace your custom recovery automatically during first boot. To prevent this, use Google to find the proper key combo to enter recovery. After typing fastboot reboot, hold the key combo and boot to TWRP. Once TWRP is booted, TWRP will patch the stock ROM to prevent the stock ROM from replacing TWRP. If you don't follow this step, you will have to repeat the install.