I'm reading a book by Horace Walpole called The Castle of Otranto now. And I found the last part of this long sentence confusing:
Yet whether provoked at the peasant having observed the resemblance between the two helmets, and thereby led to the farther discovery of the absence of that in the church; or wishing to bury any such rumour under so impertinent a supposition; he [Manfred] gravely pronounced that the young man was certainly a necromancer, and that till the church could take cognizance of the affair, he [Manfred] would have the Magician, whom they had thus detected, kept prisoner under the helmet itself, which he ordered his attendants to raise, and place the young man under it; declaring he should be kept there without food, with which his own infernal art might furnish him.
To me "with which" (in the last line) is confusing.
I understood this sentence as "His own infernal art might furnish him with food" and "with which" refers to food...
But I don't understand this usage logically because when I see the meaning, it's like saying "He should be kept there without food, which infernal art might give him." but why would he put him there when his infernal art could give him food anyway?
Maybe I'm not interpreting it correctly, and that's why it doesn't make sense. So I want your help now.