To prevent it from being deleted again, add the game folder to your antivirus exclusion list.
: Malware can sometimes masquerade as legitimate DLLs. Running a full virus scan on your computer can help determine if this is the case.
Always obtain DLLs via official installers or trusted community packages.
At its core, a DLL file like contains instructions and data that multiple programs can use simultaneously to perform specific tasks. This particular file is typically bundled with "Online Fix" patches, which allow users to access online multiplayer or co-op modes in games that have been modified to bypass digital rights management (DRM).
If you are unsure about a file, run it in a virtual machine or use tools like VirusTotal to scan the specific DLL for known threats.