I'm on my first Azure Moon playthrough of Three Houses; having previously played Crimson Flower and Verdant Wind in that order, and I recently completed Mercedes' paralogue and got the Rafail Gem and the Scythe of Sariel. What I find interesting is that the Rafail Gem resonates with the Crest of Lamine (Mercedes' and Jeritza's Crest), yet its main attribute is nullifying weapon effectiveness, which probably won't matter for Mercedes given her bane in lances and armour largely keeping her from the riding, flying, or armoured classes. It would be perfect for Jeritza, aka the Death Knight, and he's the one who gives it to Mercedes at the end of the paralogue, so why does he never use it?
I understand him not using it pre-timeskip; it would be too big a hint to his identity and it would make him even more difficult to beat than he already can be without Lysithea is. But this would be an obvious way to make him more of a threat post-timeskip in non-Crimson Flower routes that he just never uses even if you never complete the paralogue, despite the fact that there's no reason for him not to use it anymore.
There's even less reason in Crimson Flower, as the paralogue is unavailable due to Jeritza being on the same side as you. He becomes playable (admittedly only after a patch), but without the gem, even though it would be perfect for him. Why?