If you want to play vanilla Diablo II online with friends, buy Resurrected or patch to 1.14d. The battle.net servers for 1.13c are ghost towns filled with bots.
Just remember: Bring your own CD key, and leave your antivirus on. The search for the patch is safe; the downloads often are not. diablo ii lod 1.13c download
Blizzard does not officially offer a "1.13c" download. If you install from your old 2000s CDs and run the game, the updater will force you to patch to 1.14d (the final classic version). To get 1.13c, you would need to download a third-party "patch downgrader." If you want to play vanilla Diablo II