LeonarD 
The Poison King: Now that I’ve Gained Ultimate Power, the Bewitching Beauties in My Harem Can’t Get Enough of Me Volume 4 [EPUB ebook] 

After negotiations fall through with the first imperial prince Arthur, Caim and his companions are marked as enemies of the nation and forced to flee the capital of the Garnet Empire. Following Millicia’s suggestion, they decide to head east and meet with her other brother—Lance, the second imperial prince. However, on their way, they pass through a county suffering a wyvern outbreak, and Millicia’s compassion for her people leads her to ask Caim to help save them. The Poison King thus resolves to use his power for his lover’s sake.

Meanwhile, the beautiful assassin targeting the imperial family now has her sights set on Millicia! Will she be able to resist Caim’s charms and get the job done?!