In the following passage from Part 1, Chapter 2, Anthony Blanche uses hyperbole to describe the effect Lady Marchmain has on those in her circle:
"And she meanwhile keeps a small gang of enslaved and emaciated prisoners for her exclusive enjoyment. She sucks their blood. You can see the tooth marks all over Adrian Porson’s shoulders when he is bathing. And he, my dear, was the greatest, the only, poet of our time. He’s bled dry; there’s nothing left of him. There are five or six others of all ages and sexes, like wraiths following her around. They never escape once she’s had her teeth into them. It is witchcraft. There’s no other explanation."
The above passage is a prime example of Anthony Blanche's penchant for gossip and exaggeration. Lady Marchmain does not suck people dry like vampires, nor is she a witch—in fact, it is irony even to suggest this, given her devout Catholicism. She would likely be offended by such a comparison. Anthony Blanche is simply a larger-than-life character (or he wants to be, anyway). His storytelling reflects his audacious lifestyle.
On a broader societal level, Anthony's animosity towards Lady Marchmain is but one strain of rampant anti-Catholic sentiment in England. The United Kingdom's Protestant majority spent many centuries fearing their Catholic countrymen, enacting laws to oppress free expression of religious difference. Anthony's virulent villainization of Lady Marchmain reflects such deeply held sentiments.
At the beginning of Part 1, Chapter 5, Charles and Sebastian return to Oxford once the summer term has ended. Both men are less than enthusiastic about their current circumstances, depressed at the turn from summer's idle hedonism. Note the sensory imagery Waugh utilizes to communicate the pair's discontent:
Everywhere, on cobble and gravel and lawn, the leaves were falling and in the college gardens the smoke of the bonfires joined the wet river mist, drifting across the gray walls [...].
The autumnal mood possessed us both as though the riotous exuberance of June had died with the gillyflowers, whose scent at my windows now yielded to the damp leaves, smoldering in a corner of the quad. It was the first Sunday evening of term.
“I feel precisely one hundred years old,” said Sebastian.
Charles's description of autumn at Oxford parallels the melancholy Sebastian feels. Given that Sebastian is Charles's only and best friend, this melancholic mood pervades them both. The weather reflects this depressed reality: "wet river mist" hangs in the air, mingling with smoke from smouldering fires. Sebastian complains, in his hyperbolic manner, that such weather makes him feel "precisely one hundred years old." Gone is the youthful joy of his Italian summer. Now, with adult responsibilities imminent and the added pressure of maternal surveillance, Sebastian feels the full weight of age. He does not enjoy it.