The Ghost of Iki Island is a Yahata side tale where someone is going around pretending to be the Ghost. You track down the imposter near Kemono Trail and get pulled into a job to recover a smuggler's stolen navigation charts.
Part 11 of 16 in Tales of Iki
Survivors camped near Kemono Trail have heard of a Ghost on Iki. Ask them, then head to the Ghost's offering site and follow the trail of the imposter to a bandit hideout.
Rescue the fake Ghost from the bandits and lead him to a campfire. Travel together to meet his partner Fugu, then go to the Viper's Den and talk to the Viper. He needs his navigation charts back from a Mongol camp. Reach the camp outskirts, find your ally, grab the charts and return them to the Viper.
The fake Ghost is Kenji, the sake seller you know from Tsushima, up to his usual schemes.
The wiki notes a rematch quirk: if you beat the Viper in a normal playthrough, you can still fight him in New Game+ even if you decline the first time.
A Helping Hand
A Helping Hand is an Iki Island side tale where you keep running into Emotaro, a man rebuilding his home alone after the Mongols burned it. Help him four times and you get the Charm of Iron Grip.
A Mother's Law
A Mother's Law is an Iki Island side tale given by the raider leader Fune at Fune's Refuge in Yahata. It opens after Epilogue: Last Words and has you help her hit a Mongol supply ship.
A Raider's Return
A Raider's Return is an Iki Island tale where Tenzo takes Jin to meet his crew of raiders. It's how you first link up with the raiders, a group that normally has no love for samurai, in the fight against the Eagle Tribe.
Bloodletting
Bloodletting is an Iki Island side tale in Gonoura that follows Troubled Waters. You and Sugi set up a fake betrayal so Yamaneko's raiders and their Mongol allies turn on each other.