Ok so my story also happens to be a dark fantasy story, the only difference is that it's a comic. I'll drop my link here and check out if there are any other stories like this.
Name: Age I- Age of Darkness
Genre: Dark Fantasy, Action, Adventure, Mystery.
Number of chapters: 3 (divided in multiple parts, hence why it's 29 episodes on Tapas)
Plot: After a terrible tragedy throws their lives into confusion, Frederick and Nigel are forced to face their enemies in order to rescue their lost siblings from the clutches of evil. With the help of a mysterious individual, the two will discover their hidden potential and begin their journey.
Target: Mainly teens, but adults can enjoy it as well. The series is similar to a shonen manga but later on goes into seinen territory.