The Soul Is Not Required Collateral
(But it Provides Good Leverage for the Fiend)
In the MM, page 51, a demon is described as
regarding any mortals in its service as tools to use and then discard
at its whim, consigning their mortal souls to the Abyss.
As written in the Player's Handbook, the Warlock's pact does not necessarily involve the Warlock's soul for the transaction, or the relationship, that leads to powers being bestowed by the Fiend as patron. The discussion on PHB 108 and 109 declares that the patrons
expect significant favors in return.
Furthermore, the Fiend desires
the corruption or destruction of all things, ultimately including you {the warlock}
Nowhere is the bargaining described explicitly as the soul being traded for the powers. There are plenty of other ways to destroy or corrupt a Warlock. Consigning the soul to the Abyss, or making the Warlock wear a metal bikini while chained to the Demon for eternity is one way it could play out, but it isn't a necessary condition.
Were one to try and raise a Warlock whose soul was the collateral, the Fiend
May not wish to release that soul,
May consign the soul to the Abyss and then no longer cares
- May require some form of payment or other favor.
On point 3 ...
- "Bring me the heart of a virtuous priestess of (x deity)"
- "Bring me an innocent child of Prince (____)"
On point 2 ...
- An adventure to the lower planes! Mission statement: recover the soul of our party member, the Warlock.
The options are only limited by the imagination of the DM. So, work with your DM.
If you are the DM, figure out a bargain, up to and including the soul as collateral for power, and then apply leverage. That's what demons do. (The trope is as long standing as the idea of making a deal with the devil/demon for earthly power. )
For example, a Warlock whose contract is with a Fiend, per page 105, may lead a cult. In that case, the Fiend may demand/require that the Warlock feed the Fiend the souls of cult members, offering them up in some exotic or dark ritual. The Warlock's soul is not at risk so long as the quarterly payments are made. Miss a payment and you get a collection agency showing up at first, a few lower level demons, and then it gets more interesting.
As to "are there any on-line" sub question, you can take a look at reddit or a pact example like this. The favored beer in Hell is apparently Falstaff. (Taken from Marlowe's Play Dr Faustus, wherein Falstaff and Mephistopheles work a deal ...)
I see nothing there that implies it is a barrier to vision, just an active and utter absence of illumination.
You are quite right - there is nothing in the spell that says it blocks vision, just that the area is in Darkness.
However, a strict reading of normal darkness means you can't see through that (PHB p.183):
Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even most moonlit nights), within the confines of an unlit dungeon or a
subterranean vault, or in an area of magical darkness.
And a Heavily Obscured area is (PHB p.183):
A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition (see appendix A).
Which has been erratad as:
A heavily obscured area doesn’t blind you, but you are effectively blinded when you try to see something obscured by it.
So, darkness (magical or otherwise) creates a heavily obscured area. A heavily obscured area "blocks vision entirely".
Now, while it is clear what this means for "opaque fog, or dense foliage" is simple and straightforward - you can't see into this stuff and you can't see through it to stuff on the other side of it.
Applying this to darkness, however, seems to result in nonsense because, in the real world, darkness isn't a thing. In the real world darkness is the absence of light hitting your eyes from a certain direction. This can be because of an actual absence of light (underground) or because, even though the region is full of light none of it is coming your way (space). But this isn't the real world, is it?
So you have 3 options:
- Darkness works just like it says in the book - you cannot see into it or through it. This would be really cool for a gothic horror campaign even though it would make navigating at night ridiculously hard.
- Darkness (magical or not) works as it does in the real world - you can't see into it but you can see through it to illuminated areas beyond. I think this is what the rules intended even though they and the errata were poorly drafted.
- Normal darkness works like 2. Magical darkness works like 1. There is no support for this in the Darkness spell description but this is how it worked in prior editions.
Its your world - make it fun.
Best Answer
The rules don’t define this. The absence of anything saying your soul goes anywhere might indicate that it stays where it is, though—in fact, the concept of selling souls isn’t explicitly mentioned in the warlock description at all. The only use of the word “soul” in the Classes section of the Player’s Handbook is for the monk’s Diamond Soul. Actually, selling one’s soul at all is only mentioned as a possible Bond for the sage background—and it’s not given any details there, either, it just says “I sold my soul for knowledge. I hope to do great deeds and win it back.”
Anyway, since one can sell one’s soul, and the warlock description emphasizes the search for secret knowledge and everything, it seems entirely plausible that one’s soul could be one’s side of the bargain in a pact—certainly tons and tons of narrative precedent there. So we’ll say that’s what happened—so what does that mean?
Again, we have no details—it’s up to you. Traditionally, though, sold souls are collected on death, not before. For that matter, soulless characters are often not depicted as independent, sapient beings—which would make them not really viable as player characters. But as DM, you can make the warlock’s soul absent. Just be aware that this will have ramifications beyond whether this seer can see the warlock. For a fairly-obvious example, magic jar would presumably have no effect on the warlock. There will likely be others.