I was reading through my player's handbook, and got to the warlock's patrons. Then I started to wonder, is it possible to have multiple patrons? If I made a warlock character, and later in the campaign, found for example, a unicorn, could I have made a promise to the creature, in exchange for magic?
Not if they have different Pact Types
There are no rules for giving a Warlock more than one Patron. But if you wanted to define your patronage as having more than one patron of the same type (two Fiends for a Fiendish otherworldly Patron, for example), your DM might find it acceptable.
Such a decision might have an interesting narrative force (what if you were a Warlock of both Oberon and Titania, and they both use you to undermine each other), but it wouldn't be mechanically different from having a single patron in terms of the abilities given to you by your class.
But if you were attempting to gain different magical powers from two different Warlock Patrons (for example, a Fiend and a Fey), you are effectively attempting to multiclass into the same class twice for different class features. And it has been established this cannot be done.
Not as a warlock, anyway.
There are not currently any rules allowing a warlock to gain a second patron. A warlock chooses his or her patron at 1st level, and after that it’s fixed and cannot change. Many DMs would allow a swap, abandoning one patron for another, but many DMs would not allow that swap—and the rules in Player’s Handbook don’t describe any such option for warlocks.
But almost no DMs are going to allow a warlock to get a second patron: which patron you have is a major choice, and which patrons you don’t have is a major “weakness” of any given patron. The warlock class is balanced around only getting one, and not getting the other benefits.
It is possible that, at some point in the future, Wizards of the Coast will write an option like this. Maybe it will be some special Pact Boon feature combining two patrons’ boons, maybe it will be a feat, who knows. Right now, that doesn’t exist—and I would guess that most DMs wouldn’t feel comfortable trying to homebrew one for their own games. Because the choice of patron is so central to an individual warlock’s identity and power-level, any option that allowed for a second patron would have to be considered very carefully.
Finally, a lot of DMs—like those who wouldn’t allow a warlock to swap patrons—would argue that the the warlock pact is, by its very nature, impossible to change. Warlocks don’t worship patrons the same way clerics do, and the ongoing relationship between cleric and god isn’t something warlocks necessarily have with their patron. A warlock pact is a contract, and both sides are bound by its terms. Those kinds of contracts rarely have exit clauses, and you can’t hold up your end of the bargain to two patrons simultaneously. In previous editions of D&D, clerics could lose their powers by angering their god, or could choose to begin worshiping a new god—there were explicit rules for former clerics. The warlock class in those same editions, however, did not have those options, or those risks—once a warlock pact was made, nothing could change it, not even the patron.
All of that said, there is, kind of, a way you can have a warlock who finds a powerful creature, makes a pact with them, and gains new benefits from that pact: by multiclassing as a draconic sorcerer. Player’s Handbook pg. 102 says
Your innate magic comes from draconic magic that was mingled with your blood or that of your ancestors. Most often, sorcerers with this origin trace their descent back to a mighty sorcerer of ancient times who made a bargain with a dragon
This implies that while most sorcerers are due to some bargain by some ancestor, some sorcerers result from that bargain that they themselves made, rather than one some ancestor made. After all, those ancient sorcerers whose descendants make up the bulk of sorcerers today were sorcerers—there’s no reason you couldn’t do the same thing they did. And the rules, given the multiclassing variant is included at all, definitely allow a warlock with the right ability scores to start taking levels of sorcerer.