Message in Blood is Yuna Tale 4 of 4, the Act 2 finale of her storyline. After Altan strikes back at Ichi's Inn, you ride to his camp near the Kushi border and duel him. It rewards the Charm of Swift Return and 2 Silk.
Part 14 of 66 in Tales of Tsushima
Part 4 of 4 in Yuna's Tales
Find Yuna upstairs at Ichi's Inn talking with an injured Ichi. Altan got your message and wants to meet at a camp near the Kushi border. Mount up and wait for Yuna, then ride to the camp. Kill the Mongols there however you like, talk to Yuna, then duel Altan. Kill his last three guards and speak with Ichi to finish.
Find Yuna and Ichi upstairs at the inn. Get on your horse and wait for Yuna. Ride to Altan's camp and clear it. Talk to Yuna. Duel Altan, then kill his last guards. Talk to Ichi.
Altan uses a shield, so use Water Stance to break through it. Dodge and parry rather than trading hits. If you have spare Resolve, Heavenly Strike or Dance of Wrath can end it fast.
On the ride, Yuna finally explains her history with Ichi. They were slaves together on the Mamushi farm. When they escaped, Ichi fell behind screaming and was caught, and Yuna and Taka kept running. After Altan dies, Ichi says people fear the Ghost, but she accepts that Jin and Yuna did what they had to.
The Tale of Yuna
The Tale of Yuna is Yuna Tale 1 of 4, the Act 2 side tale that opens her storyline in Otsuna. You clear Mongols out of an inn run by Yuna's childhood friend Ichi and learn how to draw out the warlord Altan. It needs A New Horizon done and rewards the Charm of Rejuvenation and 2 Silk.
Silent Death
Silent Death is Yuna Tale 2 of 4, an Act 2 stealth tale at Mamushi Farmstead in Otsuna. You sneak in alone and assassinate the three Mamushi brothers without being seen. It rewards the Charm of Hidden Sight and 2 Silk.
The Black Wolf
The Black Wolf is Yuna Tale 3 of 4, an Act 2 side tale at Yoshinaka Bay in Otsuna. You and Yuna hunt the slaver who sold her as a child, clearing a coastal camp and then a Mongol ship. It rewards the Charm of Shadows and 2 Silk.
Image: Sucker Punch Productions / PlayStationFit for the Khan
Fit for the Khan is the Act 3 side tale that gets you the Mongol Commander's Armor. A blacksmith's apprentice at Jogaku Temple sends you to retake four Mongol-held spots in Kamiagata, each holding a piece of armor made for Khotun Khan.