oh my god i forgot to answer this but im gonna try not to infodump for too long, but the gist of it is that the alicorn is the ruler of a long gone kingdom in equestria which all of its habitants mysteriously died or disappeared and since then theyve used the magic to keep the kingdom going as it would before that so their magic has weakened significantly, the unicorn is a scribe sent by celestia to document the story of the kingdom and its former inhabitants before their magic runs out to assure them their kingdom and its people (ponies?) wont be forgotten ^.^