En allemand, c'est dans
Das Gelbe M mais je ne trouve pas de résumé.
En français :
Dans le laboratoire souterrain, Mortimer, se souvenant de la phrase magique du cheik Razek, lance « Par Horus demeure » au visage d'Olrik qui se retrouve déstabilisé.
https://fr.wikipedia.org/wiki/La_Marque_jauneen anglais : "For Horus remains"
Realising that he is the mastermind behind the Yellow "M", they lead a raid on his house. Septimus is about to kill Mortimer when the latter breaks the doctor's hold over Olrik by repeating the spell cast on him that caused him to lose his memories (Meanwhile, Mortimer remembering the magic phrase of Razek launched Olrik face and destabilized, a "For Horus remains" ). Suddenly going mad, Olrik turns against Septimus, chases him into the laboratory and uses a machine that vaporises the scientist.
https://
en.wikipedia.org/wiki/The_Yellow_"M"