It's not specified.
D&D 5th edition's Mordenkainen's Tome of Foes is the first sourcebook to reveal that the Ruby Rod was forced upon Asmodeus by Primus, and that devils obey their contracts out of fear of the rod's curse. It's not specified in any other 5th edition book, and the relevant sourcebooks of earlier editions make no mention of this curse either.
D&D 3rd edition's Fiendish Codex II: Tyrants of the Nine Hells defines Asmodeus, the Ruby Rod, and the rules around mortals making a pact with a creature. On page 23, regarding Faustian Pacts:
Though benefits of extraordinary value can be offered in a Faustian pact, the devil preparing it always tries to achieve the deal with a minimum possible expenditure of resources. Once the soul is securely damned, the negotiator often arranges—usually through servitors—to bring about the signatory's untimely death.
Page 25 describes what happens if a soul goes to Baator and demands the right to adjudication on the grounds that their rewards were not granted, or that the mortal was coerced or magically compelled:
The judge, usually a pit fiend, listens dispassionately to both sides and rules, as a lawful creature must, according to the law.
If the mortal wins, their soul goes free and may be restored to life with raise dead as normal. Notably, there's no mention of any curse or penalty which affects the devil who made the pact.
Asmodeus and his Ruby Rod are detailed in this book (p.155-157). There's no mention here of the rod's origin with Primus.
In AD&D 2nd edition's Planes of Law, it's suggested that devils obey their contracts because they are experts at creating and exploiting loopholes in contracts, so the contract is always in the devil's favour (p.20):
They'll cut a deal with any berk they can, and woe to the sod who agrees to their terms — and dire woe to any sod who thinks he can skirt the agreement! The baatezu've been making deals since before most Prime worlds cooled from the heat of creation; they've learned just about all the loopholes.
In Wizards Presents: Worlds and Monsters, Matthew Sernett and James Wyatt explain how and why they redefined eladrin for 4th Edition. At the time when this book was written, 4th Edition had yet to be released (and, apparently, the plural of "eladrin" was "eladrins"):
In 3rd Edition D&D, eladrins were a kind of celestial being designed by dividing up the alignment “pie” among divine servants. The warrior archons were lawful good, the animal-headed guardinals were neutral good, and eladrins got the chaotic good slice of the pie.
We knew that we no longer wanted to design monsters according to that rubric. Good-aligned creatures can be useful, but D&D simply doesn’t need that many of them. [...] Also, since our cosmology no longer hinged upon the alignment-based concept of the Great Wheel, we didn’t need to create planar inhabitants for every conceivable alignment combination.
But even though the Great Wheel, its many Outer Planes, and their various races were scrapped for 4th Edition's cosmology, the eladrin were preserved (or retconned)—now as the "banner race" for the Feywild, rather than Arborea—and as exclusively fey creatures, rather than celestials:
As we discussed what to do with [eladrin], we noted their generally fey appearance, and this led to a natural association with the Feywild. [...] Eladrins were already powerful magical beings in previous editions of the game. Now they have a very similar role, but as mysterious lords and ladies of the Feywild.
The advent of 5th Edition saw another cosmological shakeup: The Feywild, a popular element of 4th Edition, was preserved, but the Outer Planes, including Arborea, returned to the default cosmology. However, the eladrin subrace as presented in the 5E DMG, Unearthed Arcana, and finally Mordenkainen's Tome of Foes are all "creatures of the Feywild" derived from the 4th edition version.
So there are two versions of eladrin, and the difference is only a matter of what edition you're looking at: Before 4th Edition, eladrin were celestials from Arborea; beginning with 4th Edition, they're fey creatures from the Feywild.
Best Answer
Asmodeus is more ancient than the gods.
The AD&D 2nd edition book Guide to Hell (2000) details the origins and true nature of Asmodeus.
The multiverse began in chaos, and from chaos arose law. The most powerful of these were the Cosmic Serpents: Jazirian and the winged Ahriman. The two formed the undefined chaos of reality into what would be known as the Outer Planes. Each held the other's tail in their mouth, and formed an unbroken ring that defined the (then finite) scope of the planes.
But as the concepts of Good and Evil gained power, the two took sides, and fought, and each bit off the other's tail. Each drop of blood from Jazirian's tail fell and formed a couatl, but the wingless Ahriman fell into the base of the Nine Hells where he has remained ever since, biding his time until he can heal.
Ahriman took the name Asmodeus, and conducts all of his business via avatars and go-betweens. even among the gods and the lords of the Nine Hells there are none who know his true identity.
Asmodeus intentionally encourages cults of all kinds. The souls of the dead travel to the plane of their deity, or that which most closely matches their alignment if they followed no deity. But the souls of those evil peoples who reject the worship of any deity—cultists of non-divine beings, or those who reject all belief in the divine—are claimed by Asmodeus, his right as one of the Ancient Brethren who existed before there were people to believe in gods, and whose power is not reliant on faith or worship.
Asmodeus' avatar in Guide to Hell is a giant humanoid with reddish skin (not unlike many tieflings), small horns, and a finely trimmed beard. His true form is that of a serpent hundreds of miles long.
The primary source on Asmodeus in D&D 3.5 is Fiendish Codex II: Tyrants of the Nine Hells. It concurs with the AD&D description. His backstory is similar, although his relationship to the couatl god Jazirian is omitted. It is said that he bleeds perpetually from wounds.
D&D 4th edition makes Asmodeus a deity. He was already a greater deity in AD&D. Much D&D 4th edition lore is very different to earlier lore, and he was cast out by a deity he served in a manner reminiscent of the traditional story of God casting of Lucifer. Asmodeus in this edition is, as usual, described as a red-skinned humanoid with black horns and a confident, modest manner.
Asmodeus first appeared in the AD&D 1st edition Monster Manual, where he is depicted as a humanoid with small horns and a goatee beard. He is described as the most handsome, strongest and most cunning of all devils. Deities and Demigods upgrades Asmodeus to be considered a deity.
Outside of D&D 4th edition, Asmodeus has pretty consistently been described as the most powerful archdevil, and the ruler of the ninth level of Hell. The backstory with Jazirian is limited to AD&D 2nd edition (where it could be published in the year 2000 thanks to Wizards of the Coast relaxing the ban on words like "devil"), and alluded to in third edition.