What happens in Canto XIII of Dante's Inferno?

Canto XIII
The centaur named Nessus has taken his leave of the poets. Dante and Virgil tread carefully down and through the treacherous terrain of the second ring of Circle Seven. Here, Dante observes gnarled and blackened trees clinging precariously to the landscape. The leaves of these trees are “dusky” and their branches are so thickly entwined that it is difficult to say which limbs belonged to any particular tree. Instead of fruit, these tree produce poisonous thorns. The dense woods reminds Dante of the wild regions of Marmetta, located between the towns Cecina and Cornetto, near Tuscany, Italy.

“When we had put ourselves within a wood,
That was not marked by any path whatever.
Not foliage green, but of a dusky colour,
Not branches smooth, but gnarled and intertangled,
Not apple-trees were there, but thorns with poison.
Such tangled thickets have not, nor so dense,
Those savage wild beasts, that in hatred hold
'Twixt Cecina and Corneto the tilled places.”

Making the tangled nightmare even more terrifying are the Harpies who perch among and atop the trees.

"There do the hideous Harpies make their nests,
Who chased the Trojans from the Strophades,
With sad announcement of impending doom;
Broad wings have they, and necks and faces human,
And feet with claws, and their great bellies fledged;
They make laments upon the wondrous trees."

Harpies (whose name means “the snatchers”) are large birds with women’s faces, are part of the Greek myth of Phineus. Phineus, the king of Thrace, had been given the gift of prophecy, but, according to the god Zeus, over-used his power. As punishment for his transgressions, Zeus blinds the once-king. He has the man tied up and puts a never-ending buffet of delicacies before him, food which Phineus can never taste because the Harpies steal it. Eventually, the Harpies are driven away by Jason and the Argonauts. The Harpies returned to their original nesting area in the Strophades, where they found the Trojans feasting and repeatedly stole their victuals as well. One of the Harpies named Celano curses the troops, telling them that they will be so hungry that eventually, they will eat the very tables upon which they now spread their feasts. The threat is terrifying and the Trojans flee.
Dante, the author, finds the Harpies to be perfect guardians of those who have committed crimes against themselves, the suicides. These souls squandered God’s gift of life. For eternity, they will be reminded of this fact.
Mingled with the squawking of the harpies is a blood-chilling sound Dante does not recognize. Virgil tells him to break off a twig of the tree and he will discover the source of the lamentations. When Dante does so, a tortured voice cries out, the voice emanating from its trunk. The voice chastise Dante for the pain he has caused, asking him why he has no pity on the souls of men who are now transformed into trees:

“Why dost thou mangle me?"
After it had become embrowned with blood,
It recommenced its cry: "Why dost thou rend me?
Hast thou no spirit of pity whatsoever?
Men once we were, and now are changed to trees;
Indeed, thy hand should be more pitiful,
Even if the souls of serpents we had been."

Dante attends to the voice but it is difficult to understand, for like a green log on a hot fire, the words seem to be accompanied by hissing and sputtering. Stunned, Dante lets the piece of twig he had plucked fall to the ground. Virgil steps in to offer the injured shade some words of comfort, saying that his charge, new to the rules of Hell, did not know he would be inflicting pain. He also asks that the shade identify who he had been when alive:

"O thou wounded soul,
What only in my verses he has seen,
Not upon thee had he stretched forth his hand;
Whereas the thing incredible has caused me
To put him to an act which grieveth me.
But tell him who thou wast, so that by way
Of some amends thy fame he may refresh
Up in the world, to which he can return."

Moved by Virgil’s rhetoric, the imprisoned soul replies that in life he had been the private counselor to King Frederick II. Though not named in this passage, this man was Pier della Vigna. As the king’s right-hand man, della Vigna had the monarch’s complete trust. The counselor’s closeness to Frederick made many of the others in the court envious, according to the shade. In his relation of his story to the poets, the the former confidant personifies envy as a "whore." These jealous compatriots made della Vigna’s life so miserable that he committed suicide. At the end of his story, the former counselor asks that when Dante returns to the living world, he restore della Vigna’s reputation.

“I am the one who both keys had in keeping
Of Frederick's heart, and turned them to and fro
So softly in unlocking and in locking,
That from his secrets most men I withheld;
Fidelity I bore the glorious office
So great, I lost thereby my sleep and pulses.
The courtesan who never from the dwelling
Of Caesar turned aside her strumpet eyes,
Death universal and the vice of courts,
Inflamed against me all the other minds,
And they, inflamed, did so inflame Augustus,
That my glad honours turned to dismal mournings.
My spirit, in disdainful exultation,
Thinking by dying to escape disdain,
Made me unjust against myself, the just.
I, by the roots unwonted of this wood,
Do swear to you that never broke I faith
Unto my lord, who was so worthy of honour;
And to the world if one of you return,
Let him my memory comfort, which is lying
Still prostrate from the blow that envy dealt it."

Dante is so taken aback by Pier della Vigna’s story he is unable to speak. Virgil, therefore, asks the shade to explain two things to Dante, if there is any hope of the living poet to do what the shade asks. First, he says, he must explain how souls become trees. Then he must say if he knows of any way these trapped souls can ever be released.
Realizing this is likely his only opportunity to clear his name, the shade complies. In a voice that is more wind than man, della Vigna explains that the souls had been flung into the second ring of Circle Seven by Minos. With no ability to secure themselves, they root wherever they land. As they grow and become saplings, the Harpies continually eat them. The tortured, della Vigna says, long for their earthly bodies but this is their punishment for the sin of suicide. They did not appreciate their corporeal forms in life but they will forever appreciate what they willingly squandered for eternity; until Judgment Day, this will be the fate of the suicides. When that day comes, only a small amount of relief will be given to the tormented; only their skins will be restored. These will sit on the tree trunks:

"So may the man
Do for thee freely what thy speech implores,
Spirit incarcerate, again be pleased
To tell us in what way the soul is bound
Within these knots; and tell us, if thou canst,
If any from such members e'er is freed."
Then blew the trunk amain, and afterward
The wind was into such a voice converted:
"With brevity shall be replied to you.
When the exasperated soul abandons
The body whence it rent itself away,
Minos consigns it to the seventh abyss.
It falls into the forest, and no part
Is chosen for it; but where Fortune hurls it,
There like a grain of spelt it germinates.
It springs a sapling, and a forest tree;
The Harpies, feeding then upon its leaves,
Do pain create, and for the pain an outlet.
Like others for our spoils shall we return;
But not that any one may them revest,
For 'tis not just to have what one casts off.
Here we shall drag them, and along the dismal
Forest our bodies shall suspended be,
Each to the thorn of his molested shade."

Della Vigna’s story is interrupted by a “tumult.” There is a chase winding through the thicket of suicide-trees. Suddenly, two naked men come into view. A pack of snapping dogs pursues them. One is not able to run as quickly as the other. The faster man calls the slower man “Lano.”
“Lano” was a member of the notorious “Spendthrift Club.” The members of this club were the wealthy sons of Italian elites who made it their mission to squander their inheritances on frivolous pleasures. The faster man taunts Lano for his lagging behind. But by not paying proper attention to where he was going and taking the time to tease Lano, the man stumbles. This is enough time for the hounds to descend and tear them man to pieces.
The man has fallen into a thorny bush. The bush, having many of its branches broken, cries out in pain. The reader learns the man’s name is Jacopo Rusticiucci, of Santa Andrea, Italy. In life, Rusticuicci had been a member of the Ghelph party, and allegedly a sodomite.
Virgil feels pity for the injured bush. He asks the trapped shade what his name had been in life. The shade replies that he had no name that would be recognizable; that he had simply been an ordinary Florentine. Despite his lack of importance in life, the shade predicts that Florence will never know peace. He claims that the endless unrest is due to John the Baptist, who, because he brought Christianity to Italy, displaced Mars, the Florentine god. Mars, the god of war, will forever seek revenge and never let Florence rest:

"O souls, that hither come
To look upon the shameful massacre
That has so rent away from me my leaves,
Gather them up beneath the dismal bush;
I of that city was which to the Baptist
Changed its first patron, wherefore he for this
Forever with his art will make it sad.
And were it not that on the pass of Arno
Some glimpses of him are remaining still,
Those citizens, who afterwards rebuilt it
Upon the ashes left by Attila"

At the end of his story, the shade admits that he is in Circle Seven because he too had committed suicide:

"In vain had caused their labour to be done.
Of my own house I made myself a gibbet."