Not a Day for Saints by Stranger aka BT: https://archiveofourown.org/works/94039
Stranger was one of the earlier writers in the fandom, and her stories are really well written. A nice K/D Valentine’s day story.
Confidential by Kadorienne: https://archiveofourown.org/works/99244
Eroica has been stealing top secret information. NATO believes he has been turned. Klaus is ordered to determine what Eroica really is up to - by way of seduction if necessary. But what is really the truth?
Getting Married Is A Hell In Making by Chemcat: https://archiveofourown.org/series/58758
Dorian's worst nightmare come true: he gets invited to Klaus's wedding. But there might still be a possibility of things ending well.
Silent Night by Kat and Whizzy: https://archiveofourown.org/series/1428655
Christmas at Schloss Eberbach. Klaus is working on a piano piece he's composing when Eroica shows up. Which could have led to blood shed, but Dorian is behaving and it turns into a fairly nice evening for both involved. A few Christmas later, Dorian returns - and this time Klaus is prepared for him.