It's obvious that the witches are in control of the cauldron. Why would "gods" who imprisoned the evil king give it to them? For that matter, where are these gods during all this mess? Speaking of the witches... They sure seem way too powerful for "simple" witches. These creatures can shake the core of the earth, project themselves into the sky, BRING BACK CREATURES FROM THE DEAD ! ! ! Seriously... Why didn't they just ask the witches to destroy The Horned King for the sword. They seem to be WAAAY more powerful than that guy! Finally, why in the world would "the gods" (or whoever) give the gift of sight to a pig??? The pig don't understand what it sees and no one would pay attention to it anyway. And speaking of the pig, it doesn't really see any future. All it sees is that The Horned King is after it or the cauldron and at the end that our heroes are ok. What's "future" about that?
If you're looking for an in-movie explanation, I can't give you one. I highly suggest you read the books, but since I sometimes wonder how many readers still exist in the world, I'll just give you a spoiler-filled explanation.
The reason why this movie makes no sense is because Disney butchered the source material. It attempted to combine the first two books in the Chronicles of Prydain series, and it just kind of made things up as it went. For example, the Horned King isn't even in The Black Cauldron, he had no dialogue, and he certainly wasn't seeking the cauldron. So, you can already tell how closely Disney is following the source material. Here's the story of the cauldron in the books.
Basically, there's this guy known as Arawn, the Death Lord, and his domain lies in a land known as Annuvin. He bargained for the cauldron from the witches and used it to create the Cauldron-born. Unfortunately, he didn't return it on time so the witches stole it back, and that's originally why they have it. It has nothing to do with "gods", which is something Disney made up entirely, and it has nothing to do with the Horned King, who was little more than a minion of Arawn in the first book.
Yes, the witches are very powerful, but in the book they don't bring anyone back from the dead, cannot project themselves into the sky, or any of that other stuff. The Horned King can only be destroyed by someone who knows his true name, and there's only one character in the books who knows what it is, which brings me to...
Hen-Wen, the oracular pig! In the book, Hen-Wen doesn't have any knowledge of the cauldron, but since Hen-Wen originally dwelled in Annuvin with Arawn, she knows the Horned King's name. We don't really see these in action, but she uses "letter sticks" to "talk" if someone uses her abilities. I suppose she spells out what she wants to say, which is quite a bit different from the movie's portrayal. Anyway, Hen-Wen was originally sought after by a character Disney cut from the film (they cut over 9 characters from the story, many of them essential) who was seeking out the Horned King's name so he could destroy him.
So, hopefully that clears everything up. It doesn't really, because the movie is so radically different from the books that the explanation doesn't make sense within the context of the film, but it's better than nothing, right?