Legally, you can download the client directly from Blizzard, but you must own a valid license key to play. While unofficial "free" downloads exist on archival sites, they often lack the keys required for official Battle.net play. Official Purchase and Download
Your 26-character digital license keys are listed on that same account page; you will need to enter these during the installation process. Popular Ways to Play Today Diablo 2: Lord of Destruction Free Download
Once purchased, the installers are available in your Battle.net Account Settings under the Games & Subscriptions section. Legally, you can download the client directly from