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This question is inspired by this question. In that question, one of the answer explains that you can predict if the soul is willing or not by means of some divination spells. It, however, doesn't address (so this question sparks):

How to persuade an unwilling soul to become willing to be resurrected again?

Answer that explain how to do this as PC is surely better than those that will only works as DM narrative/fiat.

I'm unfamiliar with after-life lore in Forgotten Lore setting, so if you think that will help explaining the answer, please do. If I'm not mistaken, I've read that a lawful good soul usually won't be willing to return because it's reached Valhalla or something.

Method 1: Have someone else raise them

Dungeon Master's Guide p.24, Bringing Back the Dead, suggests this exact course of action:

A soul can't be returned to life if it doesn't wish to be. A soul knows the name, alignment, and patron deity (if any) of the character attempting to revive it and might refuse to return on that basis. ... If the evil cleric wants to revive Sturm to interrogate him, she needs to find some way to trick his soul, such as duping a good cleric into raising him and then captureing him once he is alive again.

Method 2: Ask them in person

When a creature dies, its soul departs its body, leaves the Material Plane, travels through the Astral Plane, and goes on to abide on the plane where the creature's deity resides.

In most campaign worlds, planes are somewhere you can travel to. If you can find the soul and communicate with them, you may be able to change their mind. Finding them may be a challenge, and whether or not you can communicate with disembodied souls on their plane is up to the DM.

In some cases, souls don't pass on correctly, remaining as a kind of undead, and are much easier to find. This is especially convenient if you want to raise an enemy who returns as a ghost or revenant who will haunt or hunt you, so you don't even need to do much work to find them.

Method 3: Summon them

According to Monster Manual p.67:

When the soul of an evil mortal sinks into the Nine Hells, it takes on the physical form of a wretched lemure.

It's not clear as to how long this transformation takes. It does describe lemures as twisted and tormented souls, which suggests that it may take some time, and devils are described as patient immortals who don't mind waiting for years.

Still, it is possible that your target will have transformed into a lemure by the time you have com, at which point you may be able to summon them with any spell which would summon a lemure, or, if it has been promoted, a higher-ranking devil. You may then be able to have a chat and try to convince them.

Method 4: Wish

Wish can do anything, although naturally this is a last resort.

Method 5: Divine intervention

Again, this is a last resort that can do anything, but your unwilling soul might come around to your way of thinking if their deity puts in a good word for you.

It depends on why they are unwilling

The Dungeon Master's Guide (DMG) provides some guidance on this issue (DMG, p. 24, bold added):

A soul can't be returned to life if it doesn't wish to
be. A soul knows the name, alignment, and patron
deity (if any) of the character attempting to revive it and might refuse to return on that basis. For example, if
the honorable knight Sturm Brightblade is slain and a high priestess of Takhisis (god of evil dragons) grabs
his body, Sturm might not wish to be raised from the dead by her. Any attempts she makes to revive him automatically fail. If the evil cleric wants to revive Sturm to interrogate him, she needs to find some way to trick his soul, such as duping a good cleric into raising him and then capturing him once he is alive again.

So if the thing making a soul unwilling is the name, alignment, and/or patron deity of the spellcaster attempting to ressurect them, the safest workaround would be to enlist the aide of a spellcaster the soul would be more well disposed to.

However, some souls may be unwilling for other reasons. Perhaps they are satisfied with their work on earth, and ready to enjoy their eternal rest. Perhaps the afterlife has offered them something they cannot attain on earth and has become a higher priority for them. Or perhaps they were uncomfortable on earth, troubled by age and infirmity, and are glad to be rid of it.

In these situations, you may have a harder time. In the last case, you could attempt to cast a different spell, such as Reincarnate, which would give them a younger healthy form. In other cases, you may need to somehow convince them that the cause for which you are attempting to resurrect them is worthy, or that your need is great (and is sufficiently in line with theirs).

This may be difficult, as no resurrection spell gives you the ability to speak to a soul and attempt to coax it back to life. Depending on your DM's flexibility, you may be able to use the spell Contact Other Plane for this purpose (PHB, p. 226-227, bold added):

Contact Other Plane You mentally contact a demigod, the spirit of a long-dead sage, or some other mysterious entity from another plane.

This tells us that the spell can conceivably contact the spirits of the dead. The spell does not specifically say that you can choose who you are contacting in this way, but it also does not forbid it. Your DM will have to decide.

Furthermore, the spell Contact Other Plane does not explicitly permit you to have a conversation with the entity you choose to contact. But you can ask it questions: and these questions may be worded in such a way as to impart or gain relevant information, if you wish (E.G. First question "Would you be willing to come back if you knew that the fate of the world was at stake?" 2nd Question "Did you know that the reason we are trying to resurrect you is that the fate of the world is at stake and you're the only one who could save it?").

Ultimately, the way to make a soul willing to return is the same as the way to make anyone do anything: find out why they don't want to and try and remove it, or find out what they do want and see if you can offer it. Depending on the soul and its reasons, this may be easier or harder (or possibly impossible) to do.

Other great answers here already. I just wanted to add how it works in Matt Mercer's Critical Role campaigns, as they have a unique, home-brewed, resurrection ritual to convince a soul to return and bring PCs back to life. I haven't seen them use it for an NPC, but I suppose it could work for them too.

1. First, some part of the body of the dead PC must be recovered. The more recently they died the better in terms of DC to revive them.




2. Three other PCs must perform an action to persuade the soul to come back. Sometimes this is sacrificing a valuable or sentimental object. Other times its reminiscing about a shared memory and calling to the soul of the deceased to make more memories with them in the future. Sometimes its a song, joke or poem.




3. Each of the three PCs (you could lower it to 1 or 2 depending on the size of your party) that performed an action must make a skill check according to what type of action they did. Persuasion for being logical, performance for a song, whatever the DM deems is appropriate.




4. The DM makes a straight roll. The DC of which is lowered by successful rolls of the 3 players and increased by how many times the PC has died in the past and how long its been since the PC died.




5. If the DM's roll succeeds, the player whose character died may then choose if the 3 PC's actions were convincing enough to bring their soul back. As you can see, the make a whole personalized ritual out of it.

Cannot Be Done

This is how I interpret this question: How can I persuade an unwilling soul to be revived? If the soul is unwilling to be revived for any reason, then the spell will fail, as only wiling souls can be revived. Unless you have means of talking to the soul, or, you might be able to dupe a cleric of whatever alignment into doing the revival for you that way. However, if the soul is simply unwilling to be revived, then it Cannot be done!

Without the use of some homebrew content, I really do not think that that would be possible. Usually, whenever I play, and when someone is attempting to revive someone, it is simply either they do or don't want to be revived. We never really tried talking to the souls, nor would I have any idea on how one would go about doing such a thing, but... Yeah. Sorry, I do not believe that it can be done.

On further research, I have found a way that the PC may be able to find out Why the unwilling soul is unwilling. If you can find the corpse, you could cast Speak with Dead and then pose the question "Would you be willing to be revived during death?", then follow up with asking the corpse "Why (or why would you not) would you want to be revived? However, that knowledge that you may glean from the corpse is only from the time it died, and events may have transpired that may have changed it's mind.

At the most, all you would be able to is to gain information on why the soul might be unwilling. Anything after that is case specific.