Nachtmusik Another ~Joshuu Kishi Ceres ni Midara na Rakuin o Kizamu~

Alec, the royal tutor, was gradually making progress in his training of the three princesses.
One day, a mysterious female knight suddenly attacked the estate. Her name was Celes.
She had come alone to rescue Princess Phyllia.
However, she was swiftly captured by the maid, Millie, with almost no resistance.
Though she refused to talk during interrogation, Alec’s connections quickly uncovered her identity.
Normally, Alec would execute any threat immediately, but for some reason, this knight intrigued him.
And so, instead of ending her life, he began a feast of lust with her as the main guest of honor…

[Translated from dlsite]