Word for "object of malignant joy"
Solution 1:
A thrall is a reasonable match. Wiktionary surprisingly lists the archaic/literary count/concrete usage first:
thrall
(1) One who is enslaved or under mind control.
(2) (uncountable) The state of being under the control of another person. [He was in thrall to the Dark Lord']
Lexico gives the order of idiomaticity correctly, and adds register caveats:
thrall [noun] 1 literary The state of being in someone's power or having great power over someone.
- the town was in thrall to a villain
___ 1.1 historical A slave, servant, or captive. ...
- Later that night, the two flew into the village and laid waste to it, killing some people while making thralls of others.
An example from a fantasy website, JonnyAU on Reddit:
- Gollum was a thrall of the ring, and thereby a thrall of Sauron.
Solution 2:
"Plaything" is actually the best match to my mind. It dates from the 1670s, so I hardly think that too modern for the Victorian tone of the sentence. The nuance of malignancy may not be intrinsic to the word, but the fact that it is demons who are doing the playing makes that connotation redundant anyway, in my view.
Another possible alternative to the offerings already suggested here is "pawn". The word does imply being played with, though perhaps more with a strategic goal in mind than for pure malevolent pleasure.
"Thrall": nice, but does not suggest play.
"Puppet": also good, but suggests a total loss of agency.
EDIT: and one more idea came to me last night which I think I like most of all: sport. I'm actually surprised it hasn't been mentioned yet:
He gave himself over to riotous living and became the sport of demons.
In this sentence sport retains its original meaning of pastime or entertainment and is a shortening of "disport", which could also be used for more archaic effect.
Solution 3:
In my opinion there is no single word term that expresses the combination of ideas "under the evil power of some entity that derives malignant joy from its use over you".
There is something that comes close in modern English and that is not "unliterary"; I am thinking of the word "punchbag" (British English; "punching bag" in American English). The vicious enjoyment is not explicitly formulated but is nevertheless often a felt direct consequence in any situation where it is used.
(Cambridge Dictionary) punchbag a person or thing who is severely criticized or punished, as a way for other people to express angry feelings
♦ Government workers have often been the punching bag of politicians, comedians, and the news media.
♦ He insists the industry is changing after a decade of being everyone's favourite punching bag.
(Wiktionary) 2. (figuratively) A person serving as an object of abuse.
Solution 4:
puppet [M-W]
-
a: a small-scale figure (as of a person or animal) usually with a cloth body and hollow head that fits over and is moved by the hand
b: MARIONETTE
-
DOLL sense 1
-
one whose acts are controlled by an outside force or influence
—a puppet ruler
He gave himself over to riotous living and became the puppet of demons.
Or, less literary, pop-culture proper noun:
He gave himself over to riotous living and became the Good Guy of demons.
BEFORE:
AFTER:
Solution 5:
pincushion 2. An object of frequent criticism or hurtful treatment Freedictionary (ahdictionary)
It obviously emphasizes the sadistic physicality you mention and more indirectly perhaps "malignant joy" as well.
Newman swallowed hard. So this is the final sendoff in return for a lifetime of service, he thought. No gold watch, no cake, no thank you, just the opportunity to act as a pincushion for whatever horrific fantasy the military scientists have cooked up this year. Live Bomb and the Pink Platoon (2012)
The "horrid game" of Hunt the Pincushion, whose sole object is to torture the smallest, weakest, fattest, most crippled, or other ostracized child... Forbidden Journeys: Fairy Tales and Fantasies by Victorian Writers (2014) p.320