ntdll.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 lowest-level user-mode component of Windows, sitting directly above the kernel. Almost every system call passes through it.
Where you see it: Application crashes reported as "faulting module ntdll.dll", which is one of the least specific error messages Windows produces.
A crash blamed on ntdll.dll rarely means ntdll is at fault. It is where the failure surfaced, not where it started. In practice the usual causes are faulty RAM, an unstable overclock, a broken driver, or an application bug. Run "sfc /scannow", test memory with Windows Memory Diagnostic, and update or roll back recent drivers. Do not download a replacement: the file is version-locked to your exact Windows build and a mismatched one will not boot.
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.