So in my current campaign, my PC's are doing really well and we are all having fun...but I have a really cool narrative idea but in order for it work I need to kill one of the PC's, who has 70HP @7lvl...

Their strength is like 18-19 as well...

Was thinking "spawn of kyuss" or some sort of beefed shadow to drain strength 

Be very wary of any idea you have that involves killing a player. I was in a game once where the DM singled out and killed a player's character in order to advance the plot, and even though the dm's plan includes the player's character getting resurrected, the player was still upset and felt horribly that the character he had a strong concept for and a lot of investment in could just be torn down for plot reasons through no fault of his own. It lead to an inter-party conflict that nearly ended the campaign.

As a rule, no matter how cool you think it would be, players hate having their autonomy taken away, especially when they feel like they're being punished for something that's not their fault, or that they've lost without being given the opportunity to win. 

If you want, you as the DM can have a contingency plan on hand for if a player character dies, but I would absolutely not single out a player for death unless that player knows what they're getting into and is willing to risk it by their choice, or unless you've taken that player aside out of game and asked them if they'd be cool with their character dying for narrative purposes. 

Writ large, player death should only happen as a result is decisions the players make;  poor strategy, use of resources, didn't run, heroic last stand, etc. Those can all lead to epic moments when the players know what they're getting into, but spring death on them with no prior discussion about how common death is or isn't in your campaign? I wouldn't recommend it. 

not everybody likes to have their PC secretly marked for death as a plot convenience. actually, i think you'll find most people don't. nobody, really. but if you've got your mind set you could always claw them with a Red Slaad. just don't forget to find a way to drop some lore in-game so they know just how chest-burstingly dead they're going to be soon. it could really raise tensions if it became a race to find a cure and shockingly fall just short of a miracle.

if it's less about the certain death and more about losing a fight then, well, you've got lots of options. even dinky goblins can be dangerous if the (sacrificial) character were to, say, follow a wounded goblin with a gold coin for an eyepatch (why not!) into an ambush of poison arrows and pushed off a riverside cliff. then you can let the dice do your dirty work for you with disadvantage, drowning, and death checks.

even if the player has agreed to this (which was not made clear), the rest of the table won't expect this death. my advice is: don't cheapen the moment by simply describing it. play it out with dice.

Well they don't have to die, but I would like an encounter were death is possible, if the PC survives all good we continue on the adventure...the combat will be rolled out, and no fudging, I just want an opportunity to kill the PC, if they defeat my creature with the die, that suits me...I'm not just going to kill of the PC with fudged rolls

Also the PC's stats I listed, has a back up character that he seems eager to play

Any substantially over-CR creature can kill, if you want a decent odds of taking down a PC without taking down the entire party I'd use a monster that has mostly single target damage, particularly if it also has a means of denying line of sight. A Remorhaz is a plausible option.

I mean, literally any monster of appropriate CR for the party level can kill a character if the fight goes their way. It's just a matter of how you run the monsters...unless you're trying to kill a character in one blow. I share the hesitations on this thread about planning to drop one particular PC and would encourage you instead to consider just making an encounter that could be deadly for anyone, not one targeted PC.

For challenging fights that could lead to anyone's death, you might consider a half-dozen will-o'-wisps, two shambling mounds, or a few hell hounds. Will-o'-wisps are fast and hard to hit, and if they use their Consume Life feature on someone who is at 0hp they can potentially kill them outright and heal at the same time. I nearly took out half my level 7 players with 5 of them by using their invisibility and incorporeal movement to their advantage. Shambling mounds are tough, especially in enclosed spaces - my DM almost TPK'd our level 7 party with two of them pincering us in a sewer. As for hell hounds, their fire breath is pretty reliable AOE damage and their pack tactics make up for their modest attack bonuses.

Essentially, you don't need to have a way overpowered monster to make a fight deadly. Most CR 2 and 3 creatures, when used in groups and with maxed out HP, can cause problems for even OP tier 2 parties. Plus, if you're going for the drama and fear factor, making everyone afraid of dying instead of just one person is a great way to heighten tension and raise the stakes. 

An Abominable Yeti.

Chilling Glare (DC18 CON Save to paralyse) followed by two claw attacks auto critting if paralysed.

But like everyone else says, players don't like to feel targeted if it doesn't make sense story-wise. That said, if their STR is up around 18ish, they're probably one of the first to charge into melee which would put them right into the Yeti's claw range.

Don't feel locked into published monster stat blocks. If you design an encounter to be lethal, a few bad rolls could quickly turn into a TPK. If you instead design the encounter to be flexible, you can introduce hazards that you can bring into and out of play.

Save or suck versus reasonable DC.

Progressively more difficult skill checks that someone will eventually fail.

Waves of small monsters that eat away at their health until you bring them to the brink.

Continuous environmental damage that puts the party against a clock.

Domination effects that force the PCs to fight each other. (This is a lot of fun if the players are on board. Skip if you need to take over their character.)

Playing the statistics game can basically guarantee that someone dies without ever taking away the chance that they'll survive.

Quote from Mattlehammerqqq >>

So in my current campaign, my PC's are doing really well and we are all having fun...but I have a really cool narrative idea but in order for it work I need to kill one of the PC's, who has 70HP @7lvl...

Their strength is like 18-19 as well...

Was thinking "spawn of kyuss" or some sort of beefed shadow to drain strength 

 
Despite what you've said here I'd echo the comments about intentionally killing a PC. The narrative of the game doesn't belong to you as a DM. It belongs to the group. I've said it elsewhere, but you as DM build the world  the group tell the story essentially through 'yes and' improvisation. So the narrative might be shaped by your world, but 'planning the story' is often a sign that the DM is indulging their desire to write over the group storytelling exercise. The only other time I've known DMs to intentionally kill off characters it's when they have issues with the player rather than the character themselves. So I'd check yourself for that.

For example, I've had a player ask me to write a character out of the world. We worked together to come up with a scenario in which that would happen. Any other times I've had player deaths it's been down to really bad luck, but the players have often felt like their characters have gone out in a blaze of glory. 

However, it wouldn't be right to acknowledge that perhaps your group works in a different way. I am aware of some groups who don't care about story and essentially use D&D as an overblown minis tactical combat game. So, if it truly does suit your table and you group, you do you. 

I would suggest a fiend that has the ability to fly. Have them as one of the stronger fiends. Fly them in, or have them appear...target that specific character and have them take that character off. Either kill them outright, or fly them up and drop them from height high enough to kill the PC. That could then motivate the rest of the party to grow strong enough to take on said fiend. 

I strongly advise against intentionally trying to kill the PC without discussing it with the player first.

Also beware of TPK with too lucky monster.

And even if the player is okay with it, what about the rest of the table? Will the monster have good motivation to single out one character and not switch targets? It can feel really weird when it seems that DM is trying to kill one of your buddies. What if someone tries to sacrifice themselves to save their friend?

This is a very bad idea. 

Talk with the player before you do anything. 

This is a very bad idea.

Ok. With your PC's approval.

If melee is the thing that the PC is good at, don't get in melee.

Use 3-6 kobolds. Tucker's Kobolds.

Use traps and ranged weapons. Use poison and magic. Use fight and flee, regroup tactics.

This should put the party on notice of what any DM can do if properly motivated.

I'll reiterate what others have said: don't target any individual PC. This is their story. It's fine to throw in challenges but aiming to kill characters is just bad DM'ing. There is no scenario where it is not bad DM'ing.

If you want to up the stakes, throw tougher challenges at the party; but make it the party, not one character.

Along with multiple others. It is a bad idea to target PCs for death. It immediately changes the dynamic from a collaborative experience to DM versus players. 

I have found that using better tactics helps to provide threat. Break up multiple small groups around a terrain to fight.

If the party is on a beach next to a cliff, creatures can be: in the air, on the beach, in the water and on the cliffside. This can also pull players into having to deal with terrain mechanics of climbing, underwater movement and dealing with flight. Imagine if a tentacled creature is pulling the cleric underwater while a flying creature is carrying off the rogue. 

Use reserves, creatures can also show up on different turns and make the players have to form new frontlines against each group. 

If you have one PC that you need to single out, you need a good plot point to single them out. Not the plot after, the reason for them being singled out. And I do recommend that the player be spoken to beforehand, though you may get away with an organic feel if you're very lucky.

Here's an example - has this character felled a powerful opponent in the past? If so, have them return as a Revenant, bent on one thing only - killing this one character. This gives a plot reason for the targeted attack. Alternatively, have they wronged someone? Crossed a dragon? insulted a mob-boss? Anything that's happened in the campaign (not before it, don't use their backstory unless it explicitly states that someone wants them dead) could be a cause for them being targeted.

Another option - do they have a playstyle you can exploit to increase their risk of death? do they charge to the forefront of combat with a high initiative each time? do they hide in the back, shooting spells or arrows? exploiting their own shortcomings could lead to an organic kill with a powerful enemy, so that it doesn't feel like targeting.

Some theoretical examples:

1: You have an ambush, and every one of the kobolds shoots the target PC. This will feel crap, because they feel targeted, and should not be done.

2: The target PC always goes at the front of the group. A pitfall trap is in their path, with  constrictor snake in the hole. They have a chance to see the trap, and can avoid it if they do. If not, they will be the ones to fall in, because they always go first. This will feel much more organic.

3: The target PC always runs in and attacks, so by using a powerful enemy, they are likely to be swatted. Using an ambush as well (with initiative rolls just under the target PC) can tie up the other PCs to keep them from helping. Their decision to charge in gets them killed - organic.

4: A monster attacks the group, and only goes for the target PC. Feels crap, because the DM picked on them. Don't do this.

5: The party sees a giant, dragon, or other notably recognisable boss from the past which the target PC put the death-blow on. It is a Revenant, and goes for the target PC directly. This will feel like a puzzle (why is it picking on... oh... oh no...) and will make sense why it is picking on the target PC, and will die when the target PC is dead, because it's a Revenant.

I will re-iterate, you should talk to the player about this before you single them out, and consider what will happen if someone else ends up taking their place!

Having a plan that requires the death of a PC, especially in combat, is typically viewed as the worst sort of railroading by most players.

I mean... 
lich casts power word kill

It basically guarantees a death  but is otherwise a fairly balanced encounter. Despite the high cr if you only use that one spell as a big moment, paralyzing touch and some counter spells  then it's easily a cr 7 encounter not even difficult for the party to otherwise succeed . It's actually a great enemy to run like a witch from left 4 dead at those low- mid levels. A mad, muttering undead wizard that when disturbed freaks out and attacks players with paralyzing touch before consuming the life of one with power word kill and then probably being defeated. 

I wouldn't kill them unless you really really need to. You should probably get their approval first.

But, if you really are fine with it...

A frost salamander - https://www.dndbeyond.com/monsters/2560818-frost-salamander . They won't kill your player immediately, but they're a cool monster and should get the job done.