Previously on Over the Garden Wall: Wirt and Gregory got lost in the dark woods called the Unknown. There, Beatrice the bluebird (well, she was cursed to be a bluebird) told them she could lead them to Adelaide who could get them home...but that was a lie. After leaving Beatrice, Wirt slowly grew more and more disillusioned until Gregory was told in a dream that he was claimed by the Beast. So Greg gave himself up to the Beast instead of Wirt...
First, some things I missed: There is a Jerome Kern song called "Babes in the Wood" about dreaming, which fits well with the chapter of the same name. "Lullaby of Frogland" is probably referring to the film Lullaby of Broadway (the song came first, but it's about nightlife, while the movie is more of a soap opera). There is also a song called "Lullaby of Birdland."
And now, onto the last two chapters!
"Chapter 9: Into the Unknown"
Summary: Before they got lost in the Unknown, Wirt was a teenager in high school. On Halloween night, he finally gets up the courage to give his crush, Sara, a mixtape filled with him playing clarinet and reciting poetry...until he hears that Jason Funderburker is going to ask her out. But when she accidentally ends up with the mixtape anyway, Wirt has to take his brother Greg along with him to try and get it back. At a graveyard. At night. On Halloween.
References: The song playing at the start includes the lyrics "you're sinking like a stone," foreshadowing the end of the chapter.
We finally find out why Greg had so much candy in the first episode — he was given it by Old Lady Daniels in exchange for raking her leaves. He also stole the Rock Facts Rock from her garden.
We also find out why Wirt and Gregory are wearing their specific clothes: Wirt's Halloween costume is made from an old Santa hat and a marching band cloak, while Gregory put a teapot upside down on his head in order to be an elephant. Gregory as an elephant for Halloween makes sense considering he acts much like the Elephant's Child.
When Wirt and Greg reach the Halloween party where Sara is at and Greg runs inside, you can hear someone talking about different bat-and-ball games, including "One Old Cat, Two Old Cat," the same game that Greg plays in "Schooltown Follies" (although Greg plays it with actual cats).
The graveyard Sara and her friends go to is called "Eternal Garden." So at the end of the chapter, when Wirt and Greg jump over the graveyard's wall, they are literally going "over the garden wall."
One of the graves Greg hides behind is of Quincy Endicott, the rich tea baron from "Mad Love." Yet another indication that "the Unknown" is some form of afterlife. (Both gravestones Greg and Wirt hide behind have a skull with wings, the same skull from the title card before the show.)
Greg suggests distracting Sara's friends by pretending to be a dead elephant. A dead elephant in a graveyard would make that an elephant's graveyard. (Okay, that might be stretching it a bit. Or a lot.)
When Wirt and Greg avoid the train and fall down the hill into the lake, a folk song plays, comparing the train to death: "There's a old black train a-comin' scraping 'long the iron. You don't need no ticket, boy. It'll take you in it's time."
"Chapter 10: The Unknown"
Summary: Things finally come to a conclusion. As Greg does tasks for the Beast, Wirt and Beatrice look for him and finally have a confrontation with the very worst thing imaginable.
References: The Beast asks Gregory to bring him a "golden comb." While Greg brings him a honeycomb, an actual golden comb is a part of the Norwegian fairy tale "East of the Sun and West of the Moon." The first item that the Beast asked for was a "spool of silver thread," referencing the straw-into-golden-thread of "Rumpelstiltskin" (Greg brought him a stick with a spider's web wrapped around it).
The third task the Beast gives Gregory is to catch the sun in a China cup. Greg does this using forced perspective. While very similar to a lot of fairy tales, I can't find a specific one that involves catching the sun - however, there is a scene in Neil Gaiman's American Gods where the god Mad Sweeney takes the sun from the sky and transforms it into a golden coin.
This episode is the only one where the Beast is shown, although for the most of the time he's only in silhouette. He's tall and has antlers, much like Herne the Hunter. This passage from Shakespeare's The Merry Wives of Windsor describes Herne in much the same way the Beast was described:
Sometime a keeper here in Windsor Forest,
Doth all the winter-time, at still midnight,
Walk round about an oak, with great ragg'd horns;
And there he blasts the tree, and takes the cattle,
And makes milch-kine yield blood, and shakes a chain
In a most hideous and dreadful manner.
You have heard of such a spirit, and well you know
The superstitious idle-headed eld
Receiv'd, and did deliver to our age,
This tale of Herne the Hunter for a truth.
"Milch-kine" means "milk cows." So Herne literally made cows give blood instead of milk.
The Beast's song, "Come Wayward Souls," was based on "O Holy Night." Per Patrick McHale: "You can actually sing the lyrics of this song to the melody of 'O Holy Night' and it works." The song has the Beast saying that the "meek" would no longer have to be fearful if they "submit to the soil of the earth," i.e. die and be buried. (The use of "soil of the earth" also indicates that the Beast may be a Chthonic diety.)
The Edelwood trees that the Woodsman grinds and uses to fuel the dark lantern come from the lost souls that the Beast transforms into trees. In Greek mythology, Myrrha was transformed into a myrrh tree, Daphne was transformed into a laurel tree, and Carya was transformed into a walnut tree. (Basically, the gods loved turning people into trees.)
It's revealed that the dark lantern doesn't carry the soul of the Woodsman's daughter, but rather the Beast's own soul. There have been plenty fairy tales of men who kept their souls or hearts in safe places. It was said that Koschei the Deathless kept his soul in a needle, in a jar, in a duck, in a hare, in an iron chest, buried into a green oak tree on the island of Buyan. The same type of thing happens in the Norwegian fairy tale "The Giant Who Had No Heart in His Body," the Scottish fairy tale "The Young King of Easaidh Ruadh," and the Portuguese fairy tale "What Came of Picking Flowers."
Here's an image of what the Beast looks like in the light. He appears to be made of wood like a tree, but the body filled with holes in the shape of screaming faces.
At the end, when Greg is showing the others Jason Funderburker (the frog) to the others, his belly lights up from Auntie Whispers' magical bell, indicating that all the events that happened in the Unknown actually happened.
The last scenes show the Unknown has now passed from autumn into winter; the Woodsman is now home, where his daughter finds him; the fishing fish catches a turtle; Lorna reads the Tome of the Unknown while having tea with Auntie Whispers; Marguerite Grey looks up at the portrait of Quincy Endicott; Miss Langtree and Jimmy Bee have a date at the circus; Enoch the cat emerges from his costume; Beatrice and her dog look out the window longingly before having dinner with her family at the Old Grist Mill; Jason Funderburker the frog plays the piano; and Greg puts the Rock Facts Rock back where he took it.
And that's the end. Thanks for reading!