Fresh and Dried Serpent Viscera are the key items in the Sekiro game. You will find both of these items near the Sunken Valley. Fresh Serpent Viscera will be obtained after killing the giant snake and you will get Dried Serpent Viscera from the temple under the cave.
Follow the steps below to obtain Fresh and Dried Serpent Viscera.
How to get Fresh and Dried Serpent Viscera:
You will be located in the Senpou Temple near the Mt Kongo idol. Move out and jump down the mountain. Kill a guy down there to make his puppet. Then he will help you to fly a kite.
Once he flies the kite, you will need to go to Shugendo. Go through the cave and climb up a big tree. Jump over the valley by using the rope with the kite.
Jump over the branches of the tree to enter into the cave. Exit the cave from the opposite side and jump onto the giant snake. Attack his head and let him eat you so that he will take you to the cave.
Look for a monkey in the cave and kill him to make a puppet. He will provoke the snake and the snake will start to attack. Once he starts to attack, you can get into the temple and take Dried Serpent Viscera.
Move back to the Cavern idol in the Sunken valley. Jump on the giant snake one more time and just kill him now. You will get the Fresh Serpent Viscera after killing him.