I haven't been on the Wizards forums in a long time -- since my original (3.5/Pathfinder) group broke up. But recently got back into playing with 5E. But that means I may not know all the monsters yet.

So I have a world that has a group of threats called the Deathless. It's basically my Ravenloft but scattered around the setting rather than in an alternate dimension, though I do like the whole Shadowfell idea, so maybe slightly not in the main setting? Also, it should be noted that I still have a bit of a 3.5 mindset. Here's a monster, give it a few class levels until it does what you want it to.

For an adventure I'm planning, the PCs will be following a map to find a magic item that sort of "belongs" to one of the PCs. But a Deathless (don't know which one yet), being magic greedy, also want the item. So Boss Fight for the adventure will be against a spellcasting undead lieutenant, because the items for the PCs are protected magically. But I don't know which undead in 5E are slightly independent and can cast spells and are good for a 5th level party.

I've seen elsewhere using the Deathlock. And as I have Mordenkainen's Tome of Foes, it can work. But I am open to other suggestions.

Thanks in advance.

AJ 

Well, without knowing which books you have access to, it can be hard to recommend. 
But there is a good filtering mechanism here, go to the monsters page, click undead at the top, then set the CR range you think will be right for your PCs.

I’ll add if you mean this to be a solo fight, make sure to choose something with legendary actions. It will let the monster do things off turn and keep the fight from getting boring.

And while I, too, remember the 3e days of adding class levels, 5e is actually much simpler. Monsters don’t have to follow PC rules. So, you can always choose some enemy who doesn’t have the spellcasting feature, and slap a couple spells on them to call it good. It might adjust the CR a little, but probably not too much. 

I *can* have access to any source I need to. Another problem I have is I don't know which sources are best, since I only recently started 5E. I bought a few of the campaign settings - Ravnica, Theros,  at least-- a few other adventure books, starter kit parts 1 and 2.

I don't see it as a total solo fight, but I am thinking the minions don't take long to take out. I have only 3 players and I really don't want anyone to have to make a new character. Also, they are level 5, so a CR 4 - 6 seems right. If I use a 4, then the minions are tougher. A 6, not so much with tough minions.

Thanks for the info.

AJ

Not sure if it's what you're looking for, but the Flaming Skeleton is a CR 3 but feels a bit underwhelming as written other than the command of minions and its essentially a glowing skeleton.
Personally I would recommend the Ghast Gravecaller (2024 MM) its a CR 6 it knows 2 spells you can replace at your will if you want it to be a caster and its main attacks can cause the paralyzed and frightened conditions making it formidable. While its trait can cause Poisoned condition. Also, as the name might infer, you could have it command undead or minions if you chose to do so since it's called a Gravecaller.

Solo boss fights don't work very well in 5e (even a Hard encounter isn't particularly challenging if it's a single creature).  I suggest a ghast gravecaller supported by two normal ghasts.

Given the scenario, I'd suggest a slightly beefed-up bone naga. They can cast spells, and they're drawn to relics

Suggestion:

Whatever you choose as the monster, make sure it can actually use the item that they're in a race for. Then have its minions accessing the item overa  number of rounds (eg chipping away at the stone the sword is buried within). And if the monster actually gets the item then the tide of the battle will turn against the players.

That way the players will have to choose to either attack the main boss, battle the boss's excavating minions, etc. 

Plus it brings timelimit pressures into it. 

And makes the item more meaningful for having been hard won.

Haven't played 2014 much. But I heard of thing from there called the minion rule where you give as many creatures as you want 1hp. if they get damage they die, which reduces the amount of math you have to do (one of the main problems with multiple enemies v your players). 

I do think there's something in that. 

I imagine, depending on the level of your party, you could do something like a bunch of 1hp minions but with a damage threshold (say, equal to 1/2 to 3/4 of their normal total hp), the higher the CR desired, the bigger the threshold. If a beast takes damage = to the threshold +1 on a turn, they're toast. Otherwise they keep on keepin' on.

It'd take some playtesting to get the balance right, I imagine, but it might mean that you can alleviate some of the concerns about the power of players' action economy when you create your boss battle. 

Quote from CraigGoddard >>

Haven't played 2014 much. But I heard of thing from there called the minion rule where you give as many creatures as you want 1hp. if they get damage they die, which reduces the amount of math you have to do (one of the main problems with multiple enemies v your players). 

 Minions were mostly a 4e concept; they had math problems in 4e and would have worse math problems in 5e due to bounded accuracy.

Dragonlance, possibly? The Shadow of the Dragon Queen module has some interesting undead monsters. I would think Caradoc as an interesting boss (although I don't recall the CR), maybe dumbed-down a little if it's too powerful and/or with spells added. If you want an extra-challenging battle for your PCs, than a CR 8 (again, I'm only pretty sure) sentient anhkolox with a spell or two at its disposal might also work - they are native to the Shadowfell.