Windows Defender scan when offline

Dear all

I have a laptop which I use as an offline backup platform.

​1.
Is Windows Defender scan still effective, even though the laptop is offline?
It can’t use an online virus database / “blacklist” to compare the scanned files with.

2.
Is Windows Defender able to detect a custom made / specially developed virus?
Or does it only react to viruses that are listed / known on the “blacklist”?

Thank you