As far as I can tell, this is actually the last part of the Kara no Kyoukai event on Fate/Grand Order. After finishing all 99 missions, it unlocks a stage on top of the apartment that has Ms. Pendragon as the sole enemy.

Nice try, but the correct assassin to use here is MHX.

How rude! MHX is a Saber, she says so herself! She'll honorably stab Artoria from behind in an honorable ambush.