d3d9.dll
This file is not a system component: windows itself, not a download. Restore it from the source that installed it rather than downloading a replacement.
Do not download this DLL on its own
Searching for a missing DLL by name leads to sites that hand you that single file. It is one of the oldest and most effective malware delivery methods there is, and a system DLL from an unknown source is indistinguishable from a deliberately modified one. There is never a legitimate reason to obtain a runtime component that way.
What it is
The Direct3D 9 core library, which is part of Windows and has been since Windows XP. It is not a redistributable component and no publisher ships it separately.
Where you see it: Older games, and very often after installing a graphics mod or reshade wrapper that placed its own d3d9.dll next to the game and then broke.
Because Windows already contains this file, a "missing d3d9.dll" error almost never means the system file is gone. Either a mod placed a replacement d3d9.dll in the game folder and it is failing, in which case delete that file from the game directory, or the game wants the legacy DirectX 9 helper libraries, which the DirectX End-User Runtime provides. Downloading a d3d9.dll and dropping it into System32 is both useless and a genuine security risk.
Why there is nothing to download here
There is no verified download we can offer for this one, because installing a file is not the fix. The steps above are what actually resolves it. That distinction matters: the sites ranking for this file name are selling a download that would not have helped even if it were safe.