What happens in Canto XXI of Dante's Inferno?

Canto XXI
The dim light of the fourth pouch is virtually extinguished when Dante and Virgil reach pouch five. Dante compares the utter darkness to the “Arsenal of the Venetians” and the pitch that those warriors used to repair their ships.
They cannot see but Dante continues to proceed; Virgil cries out, “Beware! Beware!”
Heeding the warning, Dante turns just in time to see a “black devil” coming toward them. The winged demon is terrifying; even more horrific is the sight of a sinner tossed over his pointy shoulders. Held fast by his feet, the sinner is immobilized by the devil’s pitiless talons:

"Ah, how ferocious was he in his aspect!
And how he seemed to me in action ruthless,
With open wings and light upon his feet!
His shoulders, which sharp-pointed were and high,
A sinner did encumber with both haunches,
And he held clutched the sinews of the feet."

Obsessed with his prey, the demon pays the travelers no mind. Instead, before pitching the man into the boiling Phlegethon, he calls out to his fellow demons, known collectively as the “Malebranche”:

"O Malebranche,
Behold one of the elders of Saint Zita;
Plunge him beneath, for I return for others
Unto that town, which is well furnished with them.
All there are barrators, except Bonturo;
No into Yes for money there is changed."

This man, as well as the others who are tormented in this pouch, was guilty of being a corrupt politician; that is, he and his fellow sinners took money from "donors" and awarded those donors choice positions within the government. Dante compares the viciousness of the demons who set upon the sinner to that of a huge dog, a mastiff, tearing apart a thief:

“He hurled him down, and over the hard crag
Turned round, and never was a mastiff loosened
In so much hurry to pursue a thief.”
The demons haul the man under water, prodding him with like meat in a stew, never allowing him to resurface:
Not otherwise the cooks their scullions make
Immerse into the middle of the caldron
The meat with hooks, so that it may not float.
As they poke and prod the sinner, one cries out,
“Here the Santo Volto has no place!
Here swims one otherwise than in the Serchio;
Therefore, if for our gaffs thou wishest not,
Do not uplift thyself above the pitch."

The “Santo Volto” to which the demon refers is a sculpture of Christ on the cross. The sculpture was housed in cathedral of San Martino in Lucca, Italy and is said to have be created by the Nicodemus with the help of Joseph of Arimethea. Joseph owned the tomb in which Christ’s pre-resurrected body was placed. The devil also mentions the Serchio; this was a nearby river to the cathedral of San Martino in which people swam for pleasure. The demon is saying that calling out to Christ will do him no good and the river in which he is being drowned is hardly for pleasure. He then commands the sinner to not try to resist.
The travelers watch the torment in silence until Virgil orders Dante to stay hidden as the demons go about their grim work; furthermore, Virgil increases Dante’s fear by telling him that he has personally witnessed the viciousness of the demons as he “scuffled” with them on his previous journey into Hell:

"That it be not
Apparent thou art here, crouch thyself down
Behind a jag, that thou mayest have some screen;
And for no outrage that is done to me
Be thou afraid, because these things I know,
For once before was I in such a scuffle."

Dante obeys but then Virgil seemingly ignores his own advice by personally confronting the demon horde, ordering them to disarm:

"Be none of you malignant!
Before those hooks of yours lay hold of me,
Let one of you step forward, who may hear me,
And then take counsel as to grappling me."

The demons allow their leader, Malacoda,(which translates as “Evil End”) to approach Virgil; he asks the poet what he is doing in Hell. Virgil replies that he is carrying out the will of God and that the demon is beyond foolish if he thinks that Virgil and Dante can be deterred from their heavenly mission:

"Thinkest thou, Malacoda, to behold me
Advanced into this place," my Master said,
"Safe hitherto from all your skill of fence,
Without the will divine, and fate auspicious?
Let me go on, for it in Heaven is willed
That I another show this savage road."

Malacoda recognizes the veracity of Virgil’s claim and orders his army to “Strike him not” (meaning Dante) as the pair pass.
Virgil calls out to Dante, who then joins his mentor, leaving his hiding place. The demons are excited to see the living human and want to torment him:

“Wherefore I started and came swiftly to him;
And all the devils forward thrust themselves,
So that I feared they would not keep their compact.
And thus beheld I once afraid the soldiers
Who issued under safeguard from Caprona,
Seeing themselves among so many foes.
Close did I press myself with all my person
Beside my Leader, and turned not mine eyes
From off their countenance, which was not good.
They lowered their rakes, and "Wilt thou have me hit him,"
They said to one another, "on the rump?"
And answered: "Yes; see that thou nick him with it."

Malacoda, however, orders his minions to leave him be, chastising one in particular named Scamiglione. The demon then tells the poets they can pass, but the bridge they seem to be intending to use is broken:

"You can no farther go
Forward upon this crag, because is lying
All shattered, at the bottom, the sixth arch.
And if it still doth please you to go onward,
Pursue your way along upon this rock;
Near is another crag that yields a path.
Yesterday, five hours later than this hour,
One thousand and two hundred sixty-six
Years were complete, that here the way was broken.
I send in that direction some of mine
To see if any one doth air himself;
Go ye with them; for they will not be vicious."

Malacoda offers to allow some of his demons to accompany Dante and Virgil to the next passable bridge, calling them out by name: Alichino, Calcabrina, Cagnazzo, Barbariccia, Libicocco, Draghignazzo, Ciriatto, Graffiacane, Farfarello, and Rubicante:

“I send in that direction some of mine
To see if any one doth air himself;
Go ye with them; for they will not be vicious.
Step forward, Alichino and Calcabrina,"
Began he to cry out, "and thou, Cagnazzo;
And Barbariccia, do thou guide the ten.
Come forward, Libicocco and Draghignazzo,
And tusked Ciriatto and Graffiacane,
And Farfarello and mad Rubicante;
Search ye all round about the boiling pitch;
Let these be safe as far as the next crag,
That all unbroken passes o'er the dens."

This, however, is a proposal Dante, at least, is loathe to accept, especially since the demons are continuing their menacing ways:

"O me! what is it, Master, that I see?
Pray let us go," I said, "without an escort,
If thou knowest how, since for myself I ask none.
If thou art as observant as thy wont is,
Dost thou not see that they do gnash their teeth,
And with their brows are threatening woe to us?"

Virgil reassures his charge, telling him that the display is for the sinners they are employed to torment, not for them:

"I will not have thee fear;
Let them gnash on, according to their fancy,
Because they do it for those boiling wretches."

The demons and the poets go, one of them making a vulgar noise with his behind, the “trumpet of his rump.”