I recently made a level 17 barbarian hunter and I want to use him as an NPC.  The PCs at my table are only level 10 though.

I want to have him help them for one quest only (a super hard quest that is more for levels 12 or 13).  Basically, I want this quest to start off relatively easily, so that the party is blazing along, clearing out undead and dealing with beasties and then out if no where, I want an undead werewolf (my own creation) to come crashing through the forest, chased by my barbarian hunter.

The beastie will crash through the party smack bang into a thicket of thorns through which it can't pass.  It will then turn round and prepare to attack the party, just as my NPC turns up to rescue them.

I will make the beastie swipe once or twice at the party and let them hit it back but for the most part, it will be my barbarian hunter who deals with it.

He will then help them finish off the quest and make their delivery and then travel with them to the next safe town, where he will help them stock up on supplies and give them a warning about the road they plan on travelling before leaving them to continue off alone.

Later in the campaign, when the party has leveled up enough, I will have them come across this NPC again.  This time it will be him who is in trouble and the party will have a choice.  Help someone who was once kind to them and most likely once saved their skins, or leave him to his own devices.

If they help him, I will have him become a regular friend to the party at that point and help them out occasionally.  If they leave him to his own devices, he will be killed at that point and his son will eventually find out about the party and hold them responsible.

So instead of a friend and ally, they will have made an enemy.

This is my plan, but I don't know if having an NPC help out a party who are so much lower level than him is a good idea or not.

I'd like your thoughts and opinions please.

As Stormknight has said, there's a strong chance that it could simply annoy the players or send the wrong message if you don't play your cards well. 

I created a "the npc saved us" scenario in my homebrew which ran along a parallel to your idea. I had the party come across a giant mass of rat swarms, something like a tsunami of rats closing in on them in an enclosed tunnel. The party had tried to stem the tide, they ran, and they tried to find some type of safety from them. It was a harrowing experience for them, watching as all of their massive attacks were met by a wave of rats crashing down on them as dozens were stripped away by magic and ranged attacks. When they finally found a way to get out of the tunnel they were in, they blocked the opening that they had used to escape and took a moment to rest. After a couple of rounds the rats found ways to creep into the area they had holed up in, the swarms starting to converge on them, breaking down the weak structure of the walls and eventually the swarms engulfed the party yet again.

As the fight got to the point where it seemed they were going to fail I had my NPC show up. He was nameless, he was unseen, and he wiped out the entire area with one spell. After the rats were dead he walked into the area and simply stated that he felt these adventurers were too much fun to watch. He didn't want them dead, as they entertained him, and he'd be watching how they progressed in their journey. His display of power was tremendous, his attitude toward them was like a kid watching ants in an ant farm, and he foreshadowed his reappearance some time down the road. The players are still trying to figure out who he is, why he was there, and what his purpose is, some even think they know which NPC/Character I chose to introduce.

With this type of approach I waited until the players had exhausted all of their resources, they were on the brink of failure, and the aid of an NPC would be welcomed. If, however, I had brought this character in before they had even attempted to save their own skins, I feel it would have ended a lot differently. I think the players would have felt like they weren't given a chance, they had a back seat to the situation, and their role in the encounter was pointless. 

I like your idea, I hope it works out the way you want, and always remember; don't anticipate what your players will do...they'll almost always surprise you.

In my most recent Infinite Weirdos session, the party accompanied a pair of much more powerful heroes to destroy . . . Mars.  After learning they were going to blow up Mars, they were on-board—the fact that an Ancient Dead city was there was icing on the cake.

After hearing Charlemagne (one of the two heroes) comment that he could account for killing about 100-150 Ancient Dead, and Shimmer (the other) could account for about the same, they knew their power level as the party has never, successfully killed one Ancient Dead.

When Shimmer took out a very slippery, teleporting Ancient Dead with a single swipe wasn’t important.  What was important was that the party had willingly accompanied them to Mars where Overlord Koneer could manipulate events to capture one of their number and use them to destroy Mars.

The point of the session wasn’t killing Ancient Dead.  It was a trap.  It was a trap that Charlemagne and Shimmer had fallen for, and “accidentally” brought the party into the trap as well.

At the beginning of the session, the party was gung-ho for blowing up Mars.  As the session ended, and they flew away, less one of their own, Mars exploded.  Only Thane and Charlemagne watched the destruction, to witness Saeko’s sacrifice.  Everyone else was recovering from the loss.

Saeko’s Player had told me he really wanted to run a different character, he wasn’t happy with her, and wanted her to die, so she couldn’t come back.  So, her soul was used to trigger the bomb the Ancient Dead had been guarding for billions of years, unable to trigger it because they lack souls.

So, yes, it’s fine to have powerful NPCs assist the PCs, but be sure it advances the story and they don’t overshadow the players, their characters, or the story.

No advice for you, just interested, could you post the link to your undead werewolf? Would like to use that.

Quote from Yeenoghu9000 >>

No advice for you, just interested, could you post the link to your undead werewolf? Would like to use that.

That would be fine yes.  But it currently is still on paper.  I will turn it into a digital format and share it with everyone here on DDB, via the create monster part of the homebrew section.

That way it will be fair.  Everyone will get a chance to use it if they want 

The only time I use high level NPCs is to give the party guidance and to remind them not to be murder hobos. When a DM deploys a powerful NPC to save the party it just means the situation was poorly handled. I'd always give the party the option to escape or negotiate their way out.

I usually don't do this either.  But I wanted to use this to do two things.

1.  Show the party that there are other more powerful adventures than them out there

2.  Introduce an NPC that will become important in the future.  

Meeting this NPC the second time, will determine whether they have a powerful ally, or a powerful enemy.  One way or the other, they will get both though, eventually.

I was really not sure about this though, which is why I was asking.

The way you described it in the OP, it’s more like a scene that the PCs witness rather than an encounter that the PCs want to get involved in. I think it will work out the way you want it to. It would be different if the party was tracking the monster and then the barbarian comes in and appears to steal their kill.

I would just describe what happens and not roll for the werewolf and barbarian. If a PC breaks into the narrative and says “I want to ...” let them roll. I wouldn’t bother with initiative.

This reminded me of The Hobbit where they are trying to cross the mountains and they witness the stone giants throwing boulders at each other. 

I don't think you should build an encounter around the NPC, in this case.  Build the encounter for the characters.  You can describe the NPC fighting monsters/minions while the characters fight the actual encounter-based enemy.  If they are dying off; have the NPC lend a hand, until they can regain their footing.  If you build the encounter around the NPC, it takes the spotlight from the characters and just makes it seem as though you are running your mary-sue/marty-stu character.

Just my take though.

Don't do it.

Why does he have to be Lvl 17 to help? Why don't you just make him Lvl 11 and help turn the tide in a battle they're fighting that it looks like they're going to lose? I find that having a super OP character involved just makes everyone else feel like their contribution is negligible. And no one likes that.

The idea was that while travelling the party ran smack bang into the middle of a "hunt" that was way to high level for them to deal with.

Things like this happen sometimes.  We sometimes find ourselves in situations where we are powerless, only later to have the shoe be on the other foot.

I didn't do this however, I had the thing run right past them, scattering them all over the place and it threw them aside.  It didn't stop to fight them though, it just kept running.

Moments late the barbarian burst into the scene, stopped for a minute to help them up, asked if they had seen a werewolf run past and then set off in the direction the party had indicated.

I was going to leave it up to the party, whether or not they followed him, but they didn't and chose to just continue with what they were doing.

The initial encounter actually sounds pretty cool, but I would wait to see what the party does with him. They might want to 'tag along' with the barbarian, or do something else unexpected outside of leaving the barbarian. 

Having him as a recurring NPC I dont think there would be a problem with, but the idea of (if they dont help him) his son coming after them could potentially feel like it doesnt make sense or is forced, so I would be careful about how you present that circumstance. 

Thanks for your reply.  I was planning on giving them an enemy or an ally depending on the choices they made.

I still had the encounter, but left it up to the players to follow him or not.  They choose not.

Still going to have him show up more though, usually while battling beasties or enjoying the local ale and telling tales of his exploits.

He will be open to the party being his friend, like "oh hey, seen you around, pull up a chair .... Barkeep, bring me some more ale, and some for my friends here"

Or he will show up and offer them pearls of wisdom, offer them to share his camp or ask to share theirs and tell them tales and give them some advice about where they are going (he's been pretty much everywhere in his life as a soldier and then an adventurer)

Eventually, when the party are leveled enough, he will tell them about a quest he is on and ask if they want to join him.  A dangerous quest to take down a high level alpha werewolf (I am basically flavouring him as a werewolf hunter/adventurer)

So as well as their own main story, they have a side story to explore, if they choose to and I have a way of giving them a heads-up in game, when they are entering an are that might be dangerous to them.

In ye olde days the high lvl NPC Heroes were always taking care of world shattering events. They'd hire the players to take care of the more mundane tasks. As such I'd go the route of making your players feel as horrible as you can make them feel. They interrupted the hunt/ritual/investigation of the hero. As such the world shattering event is now on the lose and free to do as it pleases. Make your players feel that shame. You can even have the hero be sucked into the portal to Orcus or whatever it is to add insult to injury.

Now turn that event into a quest hook. To regain their reputation and status and self worth they now have to look into what the hero was doing. Gather allies, go on a journey and rescue this hero and/or deal with the world shattering event that is set free due to their meddling.

Here's an approach that works for me: create situations for your characters and let them (including NPCs) react to those situations as they will. I find that when I plot something out ahead of time, my players never do what I expect anyway. I've learned (too often the hard way) to avoid plotting out encounters. I have an overall plot for the campaign, but it's only an outline. The players' actions (and NPC reactions) fill in the specifics organically as we play through the situations. For the NPCs, they have a set of goals just like the characters do, and their interactions with the PCs really control how the story gets where its going. My plot outline is vague enough that this doesn't cause me problems, it opens opportunities for new hero quests and new villains, plot twists, etc.

I hope that wasn't too rambling.

Good luck!