So, I accidently killed a man that I needed to help me find the third piece of a mythical stone that I need. Is there a way that a rogue could summon a ghost or spirit without knowing any spells? I know this is probably a long shot, but he was my only lead and I need him. I know the exact place he died and I even have things of sentimental value to him if I need that. Please help. XD I have a wizard, bard, and noble in my party. Could they help me?

I'm desperate.

Hmm. Do not eat for a game week. Obtain 4 levels of exhaustion by praying forgiveness at the murder site. 

That's a question for your DM, not for forum randos.  Via their own mechanics, the only rogue I could see making any sort of allowance for would be a Phantom rogue, and you'd best hope you've taken a soul trinket from the guy.

Elsewise?  I can say that as a DM, I'd be inclined to make you sweat for it.  Losing valuable information is one of the consequences of getting murdery around folks with valuable information - if you have no access to the Speak With Dead spell I would not be inclined to make this easy on you, at all.  Time to find you an NPC cleric who can cast Speak with Dead, and then do a dangerous Adventure Favor for that cleric to earn the spell.

A Thief of a high enough level (13th+) could cast a spell from a scroll to speak with dead, as long as they had the body.

If one might find someone who could cast that spell, like a high enough level (5th+) Bard or Cleric for example, and if they could make a large enough donation of money or services to that NPC, that person might be hired/persuaded to cast such a spell for that rogue. (#sidequest)

A generous DM might also be willing to allow summon undead since you have access to personal items and the location, but there’s no guarantee that this would work, it would be entirely up to your DM. And you would still need to find a 5th+ level Warlock or Wizard for this one.

You could have them resurrected, but that’s usually more expensive than a simple speak with dead

Or you could find out which plane his soul went to and maybe go find him there. 🤷‍♂️ But I would start with local options first.

Thanks for the response @Yurei1453 ! I'm new to DnD, so I don't know all of the rules yet. The only problem with Speak with Dead is that you have to have the body, and his body was torn to shreds and is no longer viable for that particular spell which a member of my party has. I will talk more with my dm about it. I don't expect an easy pathway to the information I need, but I was just taking a shot in the dark to see if there was something that could aid me. Thanks again! 

Thanks so much @IamSposta ! I will definitely look into summon undead and ask my DM about it! I don't have access to a Warlock or Wizard, but I'm curious about ho the spell works now. Even if I don't get the chance to use it, it will be cool to learn about.

That spell is in Tasha’s Cauldron, but it’s basically the same as the playtest version [Tooltip Not Found].

https://media.wizards.com/2020/dnd/downloads/UA2020-SpellsTattoos.pdf

I'd look around the nearest temple or large city to see if it's possible to hire a cleric type character....Obviously you'd probably have to come up with a convincing lie as to why you know the location of a dead person, but it's worth a shot.

Quote from MamasToast >>

I'd look around the nearest temple or large city to see if it's possible to hire a cleric type character....Obviously you'd probably have to come up with a convincing lie as to why you know the location of a dead person, but it's worth a shot.

I don't think every god with clerics in a large city would necessarily have an objection to helping a (accidental) murderer consult his victim.  Sure "good" gods could be swayed to facilitate some sort of connection in the spirit, so to speak, of reconciliation.  On the other hand there are more neutral or evil leaning death gods who may deal with the PC for mercenary reasons, or seeing a potential mark for future exploitation.  In a similar vein don't discount warlocks, maybe you can bargain with them or they can broker a deal between the PC and the Warlock's patron (there's lots of options for making diabolic and demonic pacts, and those systems could easily be ported over to another entity like a Far Realm being etc.).  

But to the OP, yes, unless your character was a high level Phantom, the only way the rogue can contact the dead is through someone who has access to that sort of magic or metaphysics.  The options I'm presenting are sort of a "if your DM wants to" side quest that would allow the rogue and the party to explore the supernatural underground and backrooms in their city or region, start with the Rogue's guild, if they're part of one, and start asking questions about how you can pull info from the dead, maybe doors will open to the world the characters didn't know existed.

The other, more mundane option but with more player agency than simply dealing, would be old fashion detective or casing work, where the rogue explores the victims' life story see if associates may drop some hints or maybe get into their home or sanctuary and see if there's any indication where the gem piece may be found.

Any of this can happen, if you're DM and table is up to entertain the option.  If this was a fluke error and the whole table is invested in this gem quest, I'm sure they'd be interested in the story options you're proposing.  If it's more, "the dude is dead, your quest has failed" and the DM wants to move on, your effort to sway has a higher bar.

Quote from MamasToast >>

I'd look around the nearest temple or large city to see if it's possible to hire a cleric type character....Obviously you'd probably have to come up with a convincing lie as to why you know the location of a dead person, but it's worth a shot.

 This, or something similar - basically get help. No character is supposed to be able to do everything. Rogues cant speak to the dead any more than clerics can Sneak Attack (unless you're a Phantom). The game is designed to encourage you to rely on your teammates and work together to make a sum greater than its parts. Even if you are in a solo campaign, there should be NPCs that can help you with things like this.