I’m a fairly new DM, and I have a campaign idea, but I need some help with the specifics. Mainly, I need the main monster, being a mind-controller of sorts. I’ve tried finding some monsters, but it’s not going well. Does anybody have any ideas for a monster that would be able to latch on to a king, and the control them to control the kingdom? I found a puppeteer, and that’s the closest I’ve gotten. That, or some sort of influence (demonic, perhaps) that is guiding the king in what to do for it’s own gain. Anything would be helpful, thank you!

Edit: Thank you guys so much! I've checked out all of the ideas, and a lot of them could work. Currently looking farther into the vampire idea, but now I have a lot of helpful backups. Much, much appreciated. Many thanks. 

A cambion or even a mind flayer could work to suit that purpose. Additionally, an adult or elder oblex, which can create humanoid duplicates and eat the memories of humanoids, is a great antagonist, if you have Mornenkainen's Tome of Foes.

If it is near a body of water, perhaps an aboleth?

What level do you want the PCs to find this monster? 

Beholders are great for level 10ish, give or take a level or two depending on how powerful your party is, and if you include thralls you could have a beholder that has a number of slaves in positions of power doing the work for it, only having it reveal itself when the time is right.

For something slightly above a solitary beholder, you could plan an entire arc around mind flayers/elder brain stuff. I ran a campaign where a mind-flayer society was bent on taking over the surface world, and one of their main tactics was kidnapping street urchins/homeless children and killing them with intellect devourers to maintain a network of spies and assassins throughout the towns and cities of the empire. 

If it's a big level 20 battle you're after, you could plan a whole campaign around one of the demon lords, Graz'zt maybe? The lower levels of the campaign could revolve around cultists following out his orders, controlling the king on his behalf. Maybe there's a demonic cambion in the ranks, as well as incubi and succubi controlling/influencing people in positions of power all around the material plane. There's a lot you could do. 

Personal choice but I like having Devils not as necessary a big bad more, a tricky thorn in the side that shows up, a Succubus/Incubus will be good for charming and controlling the King.

Depending on the level you are wanting the players to find them you could use a doppelganger to have replaced the king, or maybe a coven of hags including  a night hag, night hags can influence dreams and nightmares, final suggestion from me could be any monster with a high enough intelligence to use a magic item that has a similar effect to what you want that gives you a much larger amount of freedom in choosing your final boss.

If you want a more fey background, you can go with a Hag coven.

If you're willing to translate stat blocks into 5e, I would recommend the Tsochar (I have seen homebrew adaptations online, though, if you're willing to do some digging), given that they seem like the perfect creature for manipulation, mostly due to their possession capabilities and unnatural nature. Other than that I would see to the usage of Neogi, as the next best thing (this could also give the king a side hustle, given that Neogi are typically slavers).