Sinh, the Slumbering Dragon

Sinh, the Slumbering Dragon is a boss in Dark Souls II. He appears in the Crown of the Sunken King DLC.

Encounter
The player first encounters Sinh after entering the white fog in Dragon's Rest. Upon going towards the center of the boss arena, Sinh will try to bodyslam the player from his nest before starting the battle proper.

Summoning
The player can summon directly in front of Sinh's boss door, where there is also a bonfire.

Lore
Sinh was a long-sleeping dragon who was found and worshiped by the people of Shulva - Sanctum City. The Sunken King built the city itself around Sinh's resting place, and created Dragon's Sanctum to protect the dragon from intruders. The Sanctum Soldiers guarded the dragon while the Sanctum Priestesses attempted to continue its slumber with their song.

However, Sir Yorgh and his powerful Drakeblood Knights eventually mounted a siege and invasion of Sanctum City in order to defeat the dragon and steal its fresh blood, which they believed granted "a true understanding of life". They were able to defeat all the guards of the Dragon's Sanctum and invade the Dragon's Rest where Yorgh attacked Sinh. Sinh proved far too powerful for Yorgh to withstand, and he was likely killed as the dragon expelled the poison it built up over the ages and completely wiped out Sanctum City, covering it in toxic poison.

It is unknown whether this poison was also the cause of the contamination of Black Gulch.