Sesshomaru finally returned to the place he had left his servants and was shocked to find them not there. That was unusual, normally they would never leave the spot he left them at, until he returned. He knew immoderately something wasn't right.
He looked around him and noticed that the trees were all dead. Even though it was winter, the trees shouldn't be dead. He walked over to one and realized that it died because of a toxic miasma.
Realizing that the miasma belonged to Naraku, he lifted his head up to try to smell in which direction the poison came from, and noticed that it was heading toward the east. He quickly ran there without a second thought, cursing himself for leaving them there alone.
He arrived at the scene and quickly smelled, both Jaken's blood and Ah Un's blood. Jaken had been ripped into two pieces and the dragon had both of it's back legs ripped out. Smelling Sesshomaru's scent in the air, the dragon locked eyes with him and an understanding was reached between them.
The demon lord walked over to the dragon and petted both of it's heads before severing the neck from the body with his sword, Tokijin. Even though Sesshomaru's eyes remained neutral, on the inside he was extremely bothered. Ah Un was a gift form Lord Sesshomaru's father and he had been with him since he was a boy. Jaken, as annoying as he was, was the most loyal servant any lord could ask for and it hurt him to lose them both, but Sesshoaru didn't have time to mourn the dead, he had to find Naraku and make sure Rin was alright.
He backtracked his steps and followed the way the miasma had gone, he flew and flew, and thought about all of the terrible things he would do to Naraku. He didn't want to think about Rin, because if he did, then he could only imagine the worst. He needed to keep his head straight and not be bothered by worry when he killed Naraku.
He arrived in a long snowy field, where the miasma was strongest and quickly realized there was a barrier protecting Naraku's castle, but suddenly, a large blue circle appeared before him with a slit in the middle, allowing him entrance.
He entered quickly, but not trying to rush. He knew Naraku would exploit any weakness he could find, so Sesshomaru walked as if he didn't have a care in the world about Rin, even though that was false.
He reached the courtyard and saw Rin's clothes on the dusty ground and his eyes instantly turned red. His mouth pushed out into a feral muzzle, but he restrained himself from transforming. Right now, he had to remain calm and not lose it.
He walked into the castle where the smell of Rin was coming from, but he also smelled Rin's blood. He flared up on the inside, as he walked down a flight of stairs. He could smell decaying human flesh, along with Kagura and Rin. The baby Kagura carried inside her also had a faint scent, reminiscent of Sesshomaru.
He opened the dungeon door and his eyes widened drastically. Inside, Kagura was naked, bruised and blooded, but he didn't care about that, what really made his eyes widen was the fact that Rin lay unconscious as Kagura was eating flesh off her arm.
If your wondering why Sesshomaru killed Ah Un it is because the dragon was in pain and could no longer walk. If a horse was to lose both of it's legs, the owner would kill it.