This is a villain I put together for a low level party (level 1-2). You’ll notice a few strange things about the way I make my custom enemies.
First of all, I agree with Matt Colville that spell lists are just too unwieldy to run for a Dungeon Master. I understand that they give you a feel for their capabilities, but in actual combat it’s too much information. Realistically, a combat encounter will be over in 3-5 rounds anyway, so chances of taking advantage of a large spell list are slim.
Instead, I pick a few signature spells and turn them into abilities. In this case, this is a very low level boss, so I had to keep it simple. The Bestow Curse gives me a little curve-ball to throw the party, and the skeleton raising mechanic (with no real basis in the rules) adds a unique element to the fight.
Toll the Dead being his primary attack, alongside one or two spells, is more than enough for the party to accept and believe this guy was a spell-caster. They never need to see this stat block and know you didn’t actually have a whole list of spells like you should have!
Creating monsters is tricky, and is based heavily on the party you are making them for. Some parties might struggle with this fight if they don’t focus on Zarken and get bogged in the skeletons. If they get the drop on him, a savvy party can drop him before he does much of anything at all.
In practice, that is what happened to me when I ran this villain for my party. They managed to sneak into his ritual chamber, get close, and then rolled high on initiative – the poor guy barely turned around before the fight was over. Sometimes that’s just how it is in Dungeons and Dragons, though. You just celebrate with the players and move on!