Especially at level 20, you need to start by explaining how your character is meant to work.
You can create darkness and fight in it, OK. But ... against creatures immune to negative energy and mind-affecting spells (e.g. undead) you have nothing, the scythe is largely decorative and incorporeal chains fails against high-CMD creatures (like anything high-CR). Against creatures which are affected by negative energy you have one attack/round for 20d6 damage which sounds good but is kind of iffy against high CRs with 200+ HP.
The save or dies/save or sucks are DC 22-23 (25-26 assuming you get a +6 headband), glancing at some CR 20's I see fort and will saves in the range +18 to +32 with an average around +24; you can't rely on them failing these.
I'd redo this character from scratch, there's no obvious way to fix it, unless I'm missing something.