unarc.dll
This file is not a system component: bundled with the installer that failed. 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
Part of FreeArc, a compression library used by installers that pack very large payloads. It is not a Windows component and no legitimate publisher asks you to install it separately.
Where you see it: Installation failures reporting "an error occurred while unpacking: archive corrupted", almost always with a large game installer.
This error means the archive failed its integrity check, not that a DLL is missing. The usual causes are an incomplete or corrupted download, insufficient disk space, or RAM errors during extraction. Re-download from the original source, verify the hash if one is published, and make sure there is ample free space. Replacing unarc.dll does not repair a damaged archive, and the download sites offering it are trading on a misunderstanding.
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.