RtlUTF8ToUnicodeN - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H
#if (PHNT_VERSION >= PHNT_WIN7)

NTSYSAPI
NTSTATUS
NTAPI
RtlUTF8ToUnicodeN(
    _Out_writes_bytes_to_(UnicodeStringMaxByteCount, *UnicodeStringActualByteCount) PWSTR UnicodeStringDestination,
    _In_ ULONG UnicodeStringMaxByteCount,
    _Out_opt_ PULONG UnicodeStringActualByteCount,
    _In_reads_bytes_(UTF8StringByteCount) PCCH UTF8StringSource,
    _In_ ULONG UTF8StringByteCount
    );

#endif
#endif

View code on GitHub

This function is documented in Windows Driver Kit here and here.