Can Anyone Explain CSM to Me?

• Mobo: ASUS Z170 Pro
• CPU: i7-6700k
• GPU: MSI GTX 970 4GD5T OC
• OS: 10.13 Supplemental
• SSD: Samsung 950 250GB - APFS format
• FileVault2: Enabled

Because I'm using a dual monitor setup, I want to disable CSM on my BIOS (since it sends the output to the wrong monitor), but my hackintosh won't even reach the FileVault2 password screen if I disable it. I've enabled -v and nothing shows up. It doesn't crash... it just goes black and nothing happens.

I've obviously tried it with CsmVideoDxe uninstalled. I've got a vanilla install with FakeSMC, AppleALC, IntelMausiEthernet, Lilu, NVidiaGraphicsFixup, USBInjectAll and CodecCommander in kexts. My hack is – as you may have guessed – a UEFI (non legacy) installation.

So what is it that I'm using which requires CSM to boot my computer?