Makutsu no Ou - Yomei Ichi Kagetsu no Doutei - Mahou Shoujo Harem wo Kizuite Ou he Kunrinsu (The King of the Demon Cave - A Virgin with One Month to Live - Build a Magical Girl Harem and Ascend to the Throne)
Based on recent community discussions and the serialization pace: Title: Makutsu no Ou - Yomei Ichi Kagetsu
In the raw release of Chapter 6.1 of Makutsu no Ou: Yomei Ichi Kagetsu no Doutei, Mahou Shoujo Harem wo Kizuite Ou he Kunrinsu Title: Makutsu no Ou - Yomei Ichi Kagetsu