Kage (OVA)

Both are hentai/action OVAs with female assassins as main characters. Both are pretty violent and have decent stories.  

Even though Basilisk isn't a hentai, it does contain nudity, and the stories both involve ninja clans. Basilisk also has a certain amount of nudity and sex scenes, however not nearly as graphic and frequent as in Shadow. 

Hentai with not just about sex, but it also has a story. Alot similarity between both anime from main character, background story, & the ending is same.  

+ Story taken place in japan 17-18 century + Fanservice everywhere (you can skip if you want) + Alot action scene + The main character is faced with a conspiracy plan 

Historical anime focused around main characters being paid for assassinations, and intrigue focused story with an underlying plot involving uncovering aspects of the opium trade. Both include a foreign presence in a Japanese city (European/Chinese). 

Both series feature strong female leads and their struggles as assassins with a heavy dose of sex and violence. 

Kage Shadow is very similar since it shares the themes of ninjas, betrayal, revenge and ships. 

Both these series are based on ninjas, assassinations and the way of the warrior. 

Samurai, ninjas, and tragedy play the main roles in both OVAs. Both feature fantastic animation and share similar art styles. 

Female ninjas/vampires, assassinations accompanied with plenty of blood and finishing moves and fantastic animation.