The base word is mercy.
The raven represents hopelessness
no.
The correct spelling is apocalypse (disaster, catastrophe).If capitalized Apocalypse, refers to the Biblical end of the world.
The prefix is un-
Pancho villa helped them by getting them food and important stuff they need by killing the rich Americans and taking their goods but not for himself but for people that needed it the most which in his case was the poor
The bird is just saying the only word it knows.
Yes, the noun 'ravens' is a common noun, the plural form of the noun 'raven', a general word for a type of bird. The common noun 'ravens' is capitalized only when it is the first word in a sentence.A proper noun is the name or title of a specific person, place, or thing; for example, Raven-Symoné (American actress) or "The Raven" a poem by Edgar Allan Poe.
She tickled the dragon unmercifully.
The whole stanza is referring to the only word the raven learned from its former "unhappy master", and that word "is its only stock and store". The former "master" spoke "dirges" (laments or mournful songs) of "hope" (read sarcasm here) often and ever increasingly of every "unmerciful Disaster" until the word nevermore became "one/melancholy burden bore" by the raven.
The opposite of merciful using a prefix is merciless.
A raven that can transform into weapons is obviously a fictional type of raven. This raven would most likely be called just that, a raven. The raven would just be recognized as a raven that can transform into weapons.