Can I contact a specific devil?

Question

I am playing a wizard who died and then made a deal with a devil to come back to life. The terms of the contract were that he brings me back, and in exchange, I give him my first born and the soul of my friend within 30 years. If I don't hold up my end, then I go back to being dead.
The devil that holds my contract happens to be Mephistopheles, and I would like to be able to contact him to renegotiate terms.

Are there any specific spells/abilities that would allow me to contact said devil to attempt to renegotiate terms?

Answer 1

Your DM may let you use Sending

Sending is a 3rd level spell that lets you send a message to "a creature with which you are familiar". You probably met Mephistopheles in order to make the deal with him in the first place, which would mean you are familiar with him. And:

You can send the message across any distance and even to other planes of existence, but if the target is on a different plane than you, there is a 5 percent chance that the message doesn’t arrive.

Emphasis mine. There's a 5 percent chance of failure when targeting a creature on another plane (Mephistopheles is most likely in Hell at any given time, I'd assume), but if you save a spell slot every day to cast Sending, you'll probably succeed after a day or two. Negotiations may require multiple castings since each one only gives 25 words to each party.

But keep in mind that your DM could easily rule that this is not a viable method for contacting an Archdevil. The DM could decide that you aren't familiar with Mephistopheles because he negotiated the contract with you via a proxy. Or that petty mortal Sendings are beneath Mephistopheles' notice, and he either does not respond to them or has some magic or ability that prevents them from reaching him in the first place. You will need to discuss with your DM whether or not this method will work.

Answer 2

Talk it out with your DM.

As a starting point I know Warlocks get their powers from other-worldly beings that they call patrons, and that they are able to communicate with them. From the "Creating a Warlock" section (PHB, pg. 106):

What kind of relationship do you have with your patron? Is it friendly, antagonistic, uneasy, or romantic? How important does your patron consider you to be? What part do you play in your patron’s plans? Do you know other servants of your patron?

How does your patron communicate with you? If you have a familiar, it might occasionally speak with your patron’s voice. Some warlocks find messages from their patrons etched on trees, mingled among tea leaves, or adrift in the clouds—messages that only the warlock can see. Other warlocks converse with their patrons in dreams or waking visions, or deal only with intermediaries.

In my current campaign I made a deal similar to yours. I decided to multi-classed as a Warlock to use the deity that saved me as my patron. I plan on getting a hold of a creatures skull and use it as my mediator to talk with him.

Weather you are a Warlock or not this information may be a good starting point to begin a conversation with your DM and work something out together that makes sense with the story and your character.

Answer 3

Canonically, in the Forgotten Realms, simply knowing its (true) name is sufficient to contact a specific devil, assuming you have access to some sort of spell which could contact a generic unknown devil.

In The Devil You Know, by Erin M. Evans, a character by the name of Bisera captured an erinyes in a binding circle, which it is established has a certain condition must be met (condition defined by the caster at time of casting) in order for the devil to be released. Towards the end of their interaction, the following exchange takes place:

The erinyes smiled at her. "So we'll have more to discuss. Now: What's the circle's condition?"

Bisera matched that wicked grin. "Tell me your name."

The erinyes tilted her head. "What if I'd told you that when you asked the first time? I'd be gone in a flash, and you'd have nothing to show for it."

"Then I'd know how to find you," Bisera pointed out. "Names have power, remember."

Now, of course, this does require you to have the ability to cast a spell to summon a devil. For example, infernal calling (XGE pg. 168), although with a strict Rules As Written interpretation, it cannot summon anything higher than CR 6. So you will need to communicate with your DM to come up with the rules for such a spell/ritual. But once you have such a spell, the way to use it to communicate with a specific devil is by knowing that devil's name.