I'm looking for a fic a read some times ago on AO3. It's set in modern GB, there are aliens invading Earth (if I remember they are the cockroach sort of aliens). Arthur chooses to go to war, while Merlin stays behind. There is this passage in which they write each other the three things they want to do when everything ends. There are also the knights, that are Arthur's soldiers and they poke fun at Merlin because Arthur talks about him.
I'll wait for an answer.