I'm new to the whole homebrew thing. For the main villain, I want him to be the equivalent of a level 20 warlock. Should I use the PC rules or a stat block? Please help.

I would design him as a PC, but that's just my two copper pieces.

What do you need to homebrew about the Warlock though?

Use a stat block. PCs are not designed to track with CR. NPCs are allowed to do things PCs can’t do. Action economy will want your NPC to have legendary or lair actions, which PCs don’t get. Also, the fight will likely last 4-5 rounds. If you make a level 20 PC, you’ll be flooded with options you won’t ever use. 

Me personally, I would start off with making the BBE as aPC, and then convert it to a monster statblock. But that’s more work than just making it as a monster, and getting the conversion right takes some experience as it’s more art than science. You would probably have an easier time starting with one of the existing warlock monster statblocks and then just adding onto it to make your BBE as tough as you want.

I agree with Xalthu, you should use a stat block you can go one of two ways:

1. A simplified PC Warlock

The Archmage is a good example of this. The Archmage is a level 18 wizard without any subclass features and without arcane recovery (which is unlikely to be used anyway unless they manage ot escape and take a short rest before the party catch up with him.

For a warlock I believe the patron is key to the build so the players should see some of the features unique to the subclass but it is likely for there to be no need to add a pact boon unless it is key to the character.A level 20 warlock knows at least 15 spells they can cast at 5th level (possibly more from invocations) but only have 4 spell slots. You can simplifything my only having 6-10 spells that cover most of what the villain ins likely to do in regards to the party.

An example of this is given here

2. A Monster NPC Warlock

As Xalthu pointed out there are a number of spells and features that can be a "win" button against a single monster or even a monster an a few irrelvent minions. If the warlock fails his save against feeblemind, combat is essentially over (unless they had a minion with the greater  retoration spell prepared). To prevent such things making what is supposed ot be a significant fight becoming trivial you may wish to give the warlock legendary resistances. Lair actions help balance action eonomy but that can also be sorted with minions.

The monster NPC warlock has features which make it clear the the NPC is warlock (e.g. eldritch invocations) but also features that are not available to PCs in order to make the fight interesting.

Thank you! The article was very helpful.

I tend to pick an existing monster statblock that is roughly correct, then adjust from there. For example, I have an NPC who is a powerful warlock, and they throw around all sorts of magic. I restricted them to combat magic and handwave useage of other magic outside of combat. The statblock shouldn't be needed outside of combat very often.

I believe I used a Mage and then upped their HP, swapped out for warlock spells, and then gave them a bunch of abilities from a couple of relevant warlock subclasses, and then sprinkled some other ones I need for plot reasons. It was fun to make.

I'd avoid using a PC statblock for them outright. A level 20 warlock with +2 Con has an average hp of 140hp. A level 8 character can reliably output 30-40 damage if they are built to fight. 4 PCs could take them out before they even act.

Ask the question in the title. What is this.