Remnants of Shimabara
Apparently, he managed to survive the arrival of the Shogun's forces when they quelled the tumultuous rebellion at Amakusa-Shimabara. He confronted Miyamoto Iori's party at his base in Koishikawa. When a Rogue Servant intervened in the conflict, he allowed Lancer to unleash her Noble Phantasm. An ecstatic look spread across his face as the area went up in flames.

Abduction
He attacked Iori's house and kidnapped Kaya. His intention for doing so is unknown.

A Warped Alliance
Not long after the Ritual began, he formed an alliance with Rider's party. He did not, however, place any trust in the serious and dutiful Shousetsu. He simply wanted a pawn that could be used to achieve his own goals.

Waxing Moon Monster
His true objective was not to take revenge on the Tokugawa family for razing his homeland, but to open this world to hell, thereby allowing him to reunite with his dead family.
He used Lancer and Kaya to transform himself into the Waxing Moon Monster and attempt to open the gates of hell, but was destroyed along with the Waxing Moon's Vessel by Saber's Noble Phantasm.
This fallen man willingly walked a bloody path in pursuit of his long-held desire. No one knows what he realized in his final moments; he did not take the hand that was offered to him.

An Unceremonious End
To Chiemon's eyes, Iori was a cold-blooded beast in the guise of a man, a demon much like himself. But because Iori conducted himself as though he were a man of virtue, Chiemon found him repugnant.
That same hatred drove him to attack Iori when he reached for the Waxing Moon, but Shousetsu stood in his way. While caught off guard, a single sword stroke ended his life.

Mutual Destruction
He moved to attack Iori as he reached for the Waxing Moon in Senso-ji, but upon seeing Iori's menacing expression, one of a true sword demon, he fled.
After escaping, he happened across Caster, with whom he engaged in battle. Under normal circumstances, a human could never overwhelm a Servant, but Chiemon's iron will brought about a once-in-a-lifetime miracle. He summoned the hellish flames that once burned a maiden to ash in a far-off land. Though he managed to fell Caster, Chiemon was also consumed by the fire.

Chiemon

Character Profile
If the Edo Period is to be defined as the era of peace achieved through overcoming the Warring States, Chiemon is the one discarded by the era. He was blueprinted to be a character that would never work as the protagonist but still shines brighter than anyone else.

Fate/Samurai Remnant has him as an independent agent who never sides with either Iori or the masterminds.

He is one of the very few survivors of the Shimabara Rebellion and learned the basics of magecraft from Mori Souiken. His lineage as a mage lasted only one short-lived generation, but he was quite effective in combat. Either he was naturally talented or he had an unknown mage ancestor.

His wish for the Holy Grail is revealed late into the story. Wholesale salvation to the abandoned ones. Even if that salvation is pointless.