The comma used here is strange but correct. The important part of the sentence is "silence its harrowing cry". However, whenever you use the word 'and' you need to make sure the two words that are linking are words that are suppose to link. Example: I kiss and hugged my mother (linked) ... as opposed to... I kiss and had a drink with my father (not linked. Therefore, not what you really want to say).
Nightmarish rituals crave a newborn. Find one, and silence its harrowing cry.
The person doesn't need to tell you to find one in this sentence. You are being told to find one as and additive or a concern that this direction must be followed. It's like telling you to jump when the only way to do it is to jump and slam dunk a basketball in the hoop (something we all know).
Find one is used like a 'henceforth' or a 'From this point' but given to you bluntly "Find one" but don't link it to "silence" because if you don't find one you can't proceed anyways.
Look at it this way:
Nightmarish rituals crave a newborn. Silence its harrowing cry. (this is all that's really needed therefore the addition was added help and grammatically correct)
English has rules. Once you learn the rules, English is easier to understand. Most people just don't know the rules and write more relaxed.