When we talked about the Asus WS X299 Pro, the comments had some grumbling about the X299 platform's lack of ECC support. Well, here's your board, boys. The WS C422 Pro/SE is the exact same motherboard as the WS X299 Pro, except that it swaps the consumer X299 chipset for a proper workstation-focused C422 offering. Throw in a compatible Skylake-W Xeon CPU (and it'll have to be a Xeon—no Core i9s are supported) and you can outfit yourself with up to 512 GB of quad-channel ECC memory.
The Xeon W-series CPUs max out at 18 cores just like the Core i9s. The high core-count Skylake-SP chips (up to 28 cores) used in Xeon Scalable parts aren't available on the LGA 2066 socket. We reckon the 18 cores of the Xeon W-2195 ought to serve most workstation needs, anyhow.
Besides the chipset change, this board is otherwise identical to the WS X299 Pro, so head over to that post if you want the details. Alternatively, you can hit up Asus' product page. The WS C422 Pro/SE doesn't seem to have shown up at e-tail yet, but it should pop up any day now. Keep an eye out if you've a mind to fit half a terabyte of RAM in your custom-built workstation.