So one of my PCs lost their friend and is looking to revive them, but I wanted to make things complicated. In order to get his soul back and revive him, they need to gather five "noble souls", which are basically high quality souls, to exchange for his soul. Kind of like in TES how there's different sizes of souls? Originally I was thinking maybe these noble souls come from champions of gods and the like, but that seems a little too...much.

Any suggestions on what kind of creatures or people would make good candidates for something like this? A note that the character who would need to collect these typically travels alone, but could contact the rest of the player characters in the campaign for help if necessary.

(Side note that I wasn't really sure where to put this, so general seemed like a good idea.)

This depends on where this requirement is coming from, and how you want the story to pan out.

For example, you might consider a noble soul to be anyone who willingly sacrifices themselves to save the character. Perhaps they need to convince people of their importance.

As another example, the 5 noble souls might have to come from innocent and pure beings, making it a lesson in the cost of resurrection and an effort from a trickery god to send the party down a road of evil, or at least seeming evil.

Simply saying that the noble souls come from anyone who is brave, true, and unquestionably "good" will make the party have to consider their moves carefully - convincing people in a lost cause to sacrifice themselves before they die, or simply following someone noble until they happen to die near their soul-vacuum. 

Added bonus points if you make it a god of trickery and when the party arrives, traumatised from killing the good people, they say "well, it would have worked if they were alive, it doesn't actually consume their soul you know!", so the players realise they maybe need to make less assumptions!

Extra alternative - bring a phylactery from a lich to the place. Bound to be some noble souls in there!

ThorukDuckSlayer - thanks for the thoughtful response! I like the moral dilemma idea, it sounds like a blast to put them through. Innocents and pure beings would do just fine for a cause like this, and it adds more of a twist too! My only issue would be that the main person seeking these souls is a Warlock who serves a reaper--so, he's actively killing people regardless of how 'good' or 'evil' they might be, it's just business to him. The rest of the party would surely be against him killing random people, but if he ends up needing their help, it'll be very interesting to see how it all pans out... [evil laughter] 

Bonus points for mentioning the lich phylactery, because the character died thanks to a Lich in the first place (and the person trying to revive him is incredibly sensitive to even the mention of them now). I've been going a little trauma-heavy on this one character in particular because there's so much opportunity for it, and he may slowly become the villain himself... (His player is loving it).

I think it would be helpful if you defined what actually want. You describe the souls as "noble", and that makes me think of people of high moral character, and is intended to create moral dilemmas and so for forth. But then you compare it to TES, where the quality of souls relates to the equivalent of the CR of the creature (with humanoids being on a par with the largest soul), which makes me wonder if you're asking about what creatures are a high enough challenge to justify trading for the PC's soul. If you could let us know what exactly you're after and details of the party (in particular, the level they ae/will be when they pursue this quest), we could probably help with more specificity. Also, if there are theme constraints, that would be helpful too.

Linklite - fair point! The "noble soul" label is kind of what I've just been calling them so far, since I'm not really sure what other phrase to use. I would imagine their CR plays a bigger role in determining the quality of their soul (after all, it's hardly a challenge if the target has an incredibly low CR, with or without combat), but their alignment / good deeds throughout life would also probably play a good bit into 'growing' it. I don't know everything about the game, so I turned to the internet to see if anyone had ideas beyond just humanoids (like the whole 'champion of a god' thing I mentioned).

The party is currently level 8, and the character (Warlock) looking to revive the dead one is seeking to do this quest ASAP, so that's where we are in terms of level. The Warlock typically travels alone, but has met up with the rest of the party a couple times and there are plans for him to do so again for story-related things. His line of work almost forces him to be more of a loner / nomad, and the rest of the party has other goals and things to take care of, so it works out. Ideally, he would be able to take out at least a couple of the targets himself (or otherwise obtain their soul, even if it doesn't necessarily mean killing them) but would require help with one or two of them, just to have some variety and ramp them up in difficulty as time goes on--this is meant to be a time-consuming, draining process that has outsiders questioning if his friend is really worth all the trouble.

I don't think there are any theme constraints, this is the first campaign I'm running and I just got into D&D recently, so everything's been kind of all over the place. How we got here is they ran into a Lich, managed to "kill" him (because they're unconventional and I realized too late that I had no idea how to use a Lich properly lol) and on his way down, he cursed a member of the party. I thought it'd be more interesting if there were conditions the Lich had to meet to revive itself in a timely manner, so if there's someone alive in their vicinity when their body is destroyed, they curse them and take their life force to restore their own in the d10 days instead of it just happening automatically. They CAN return on their own, but it takes much longer without a 'sacrifice', so to speak.

Edit: That said, the character's soul isn't exactly stuck in the Lich's phylactery, but it is stuck somewhere--which is why a typical resurrection spell won't cut it. Lucky for the Warlock, his patron was able to pull few strings, but it's gonna cost him. More than just the noble souls, I mean.

Not sure if these are all the details you'd need, but hopefully it helps! I appreciate your feedback.

Considering that you want this t obe a challenge and that the character is working for a Reaper so will not balk at killing anyone, I would make the challenge that they must gather 5 individuals to sacrifice a portion of their soul to bring back the PC. You might even try to break the PCs character into 5 different aspects (EG they love gold, they are good, they are skilled with ranged weapons, they know fire magic) and set the party the task of finding NPCs with these specific traits. 

Then I would work on future changes to make for each of these NPCs, such that when the party revisits them, they find that they have changed because of the portion of their soul that is missing - perhaps the greedy merchant is no longer greedy, the master archer is now a washed up drunk after shooting their assistant, the good farmer who always helps is now callous and careless.

Then, perhaps when they re-face the Lich (assuming the phylactery is not destroyed and the Lich is coming back), they will find out (somehow) that the Pylactery can be corrupted using incomplete souls, so they would have to sacrifice these 5 NPC's to the phylactery to cripple the Lich. This in turn might lead to the Lich desperately trying to steal the soul of the resurrected PC to complete the 5 broken souls corrupting the Phylactery, and the Party has to stop them whilst protecting the PC!

Look forward to hearing how you plan this!

ThorukDuckSlayer - I like that idea a lot! It would be really interesting to tie further into the other story things that are going on (everything is a mess, nothing was planned in advance LOL), but I'd definitely have to sit down and figure it out. Definitely a great train of thought though, thank you for the feedback :) I think the hardest part would be convincing the NPCs they should sacrifice a portion of their soul for someone they've never met... Hmm. Definitely gets the gears going!

Ok that makes sense. The tricky thing is that this concept doesn't really exist per se in D&D. That's not to say it's a bad idea - it just means that there's no ready made mechanic for working it out. As a DM, you're entitled to make these kinds of modifications. Personally, I'd just create an interesting encounter and then just use that.

Do you have the DMG and a monster compendium (usually either the Monster Manual or Monster of the Multiverse)?

If so, I'd recommend working out what CR monster would provide enough of a challenge. Work out how many characters will be on the quest, then use the DMG to figure out what rating an appropriate encounter. I'm not very experienced in this, so maybe someone with more experience can give advice. My experience has been that if going by XP given, treble the value of deadly works well for a stiff but not impossible encounter (5e has an odd system where the rating assumes that it's one of half a dozen encounters, so a single deadly encounter isn't very deadly, but 6-8 back to back ones are). Convert that to CR. I'd calculate that to be CR14 for two L8s. If someone with more experience could verify that, it would be appreciated - my experience was from lower levels, so it might not scale well.

Then go through your monster compendium and find a few interesting monsters that would provide the challenge you want. If they're weaker, you can add minions and sidekicks. If they're stronger, you can either say they were recently injured and reduce their stats, or you can have the fight happen in stages, having the monster retreat after losing so much of their health so the party has chance to rest, heal and recover.

Further thoughts on the subject:

You could consider giving the characters a magic item which can absorb part of a soul in order to reconstitute an artificial soul to fit the dead PC. Then they have to defeat (but not kill) a variety of creatures which they feel represent the PC, based on some of the things that make the PC who they are. If the Player is up for it, you can have the party decide this for themselves, and then the PC might behave similarly but slightly different based on their decisions.

But then, you have another option. The PC, now recovered, is starting to feel rough, tired. The twinkling lights on the soul-amulet they wear are fading...

...They need souls for their new Phylactery.

Linklite - thanks for all the feedback! I'll try working out a mix of humanoids and monsters as the various encounters to kind of mix and match ideas, see how that works out in theory. I'm not that great at running combat unfortunately, so something I think is going to be pretty challenging ends up being easily handled--one of our players has been playing D&D for years and is the one who taught all the rest of us LOL. I planned on sitting down and doing some planning for the next session today anyway, so I'll take a look through the books we've got and see if I can find something that feels like it'll suit what I need. 

ThorukDuckSlayer - with how things are panning out so far, if they're successful in reviving this character, he's likely going to end up in service to the same patron as the Warlock (without having to become a Warlock himself, it's just more of a "I did you a favor, now you do me some favors" kinda thing). Would be veeery interesting if they suddenly had to acquire additional souls on top of the existing contracts they have to take care of...

There are some other things that will change about the character as well if he's revived, little hints that he's not quite right...but y'know, close enough. He's still him, but time spent wherever his soul was trapped certainly did a number on him.

There are some interesting theological questions worthy of exploration with this concept - namely, are souls static or do they change within your campaign?  Under a more predetermination model, even an infant might have a noble soul, if they are an individual either destined for greatness or if they have a predetermined aptitude for greatness; under a tabula rasa model, an individual might be born with the same blank soul as others, and their choices and actions make them great; or you could have some sort of hybrid model, where a person might be born with a soul which could become a great soul, but only if they unlock that potential, or could, through force of will, overcome their own predetermination.

Each model has different implications for the quest and for the tone of your world, so worth giving the question some thought and determine what kind of world you are looking for—one more defined by fate, or one determined more by action. 

Caerwyn_Glyndwr - my personal answer to this would be that someone could be born with a soul that has the potential to become a noble soul, but if the correct actions aren't taken, it will never reach that "phase". Just like a character's actions can alter their alignment, there's a lot of factors that go into shaping someone's soul. I'd say there's only a small percentage of people who are born with a noble soul, and an even smaller percentage of those who would actually achieve one. Of course, most people wouldn't even be aware they have one, they're just living their life the way they want to. Makes for some fun if there were a prophecy of a hero who would rise, and someone decided to intervene to make sure that never happens...

Of course, if someone wanted to use the concept for their world / campaign, they might have a different answer.

This has the making to be an epic tale that is long remembered.

You said if he's revived though maybe he's not quite right...  I personally and humbly wouldn't impose any negative effects on the PC because it may be a let down after the great effort taken to bring them back.  I would give them unexpected bonuses that are relevant.  For example, they were trapped and tormented in ways nobody else around them could understand so now they are hard to frighten and get advantage on saves to fear-type spells or effects.

Anyway, I like all your ideas and think this will go over well.

paulbunyip - thanks! Glad you like them :) The character being revived isn't a PC, he's more of an NPC that travels with one of the PCs. (Since that player tends to do solo stuff partly due to conflicting schedules, they aren't often able to join the rest of the group. Thankfully, the character does a lot of stuff on their own anyway because of their line of work, but I didn't want to make them lonely, so I gave them a friend. And then he died. Whoops.) So the negative effects are, in my opinion, a-okay! If the character being revived was indeed a PC, I wouldn't have negative side effects that I determined, but the player themselves might decide something for the character, at their discretion.

The choices I've made for him is that he'll be prone to bouts of anxiety from the situation (he can't quite shake off the chill of death), and he will begin craving meat (previously he was strictly vegetarian because he's...sort of a rabbit) but it's nothing that would prevent him from functioning normally! Just some character quirks that kind of hint at the fact that he went through something. He's also more of a "soft" character, not really made for hardcore battle or anything--I think such a situation for a warrior would definitely call for them being tough beyond measure, but alas, he wasn't made for that kind of life.