By the end Buck has been transformed into a monster—“the fiend incarnate.”
Here is another Potter fiend crying over Harry Potter and the Deathly Hallows: Part 2.
Prayer is so mightful if it have its right, that it masters the fiend, and hinders him from doing his will.
Twas seen and told how an avenger survived the fiend, as was learned afar.
No, if he was a fiend on the battlefield, he was a lamb at home.
“I think von Francius would be a fiend if he could,” said Karl, comfortably.
Honesty is some fiend, and frights him hence; A many courtiers love it not.
His two aids, the Saint and the fiend, had a bad time of it.
This passionate absorption of all his energies into one channel had made a fiend of the man.
Now there was a fiend grovelling at her feet, a foul, sin-stained fiend.
Old English feond "enemy, foe," originally present participle of feogan "to hate," from Proto-Germanic *fijæjan (cf. Old Frisian fiand "enemy," Old Saxon fiond, Middle Dutch viant, Dutch vijand "enemy," Old Norse fjandi, Old High German fiant, Gothic fijands), from PIE root *pe(i)- "to blame, revile" (cf. Gothic faian "to blame;" see passion).
As spelling suggests, it was originally the opposite of friend, but the word began to be used in Old English for "Satan" (as the "enemy of mankind"), which shifted its sense to "diabolical person" (early 13c.). The old sense of the word devolved to foe, then to the imported word enemy. For spelling with -ie- see field. Meaning "devotee (of whatever is indicated)," e.g. dope fiend, is from 1865.
A devotee or user of what is indicated: camera fiend/ dope-fiend/ sex fiend (1865+)
To use a choke hold on a robbery victim: They'd take out a bodega, or fiend a few housewives (1980+ Underworld)