Once purchased, the content should download automatically. If it doesn't:
Avoid sites offering "free full game" or "DLC unlocker" downloads. These are often used to steal login credentials or infect your PC with viruses. It's always safer to wait for a seasonal sale on official platforms. Buy Call of Duty®: Black Ops III - Descent DLC - Xbox call-of-duty-black-ops-3-descent-dlc-free-download
Since the Descent DLC is paid content, the only verified way to "download" it is through official storefronts. Once purchased, the content should download automatically