Question: Princess Cheonmyeong, was a princess of Silla, one of the Three Kingdoms of Korea, and is notable as the sister of Queen Seondeok of Silla, and reigned as Silla's which number ruler?

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Jindeok of Silla: Jindeok of Silla (?  – 654), reigned as Queen of Silla, one of the Three Kingdoms of Korea, from 647 to 654.  She was the kingdom's 28th ruler, and its second reigning queen following her predecessor Queen Seondeok.  During her reign, Silla jockeyed with Baekje for favor in the Chinese Tang court.  She is also known for writing a poem of the Emperor Gaozong of Tang.

Queen Seondeok of Silla: Queen Seondeok of Silla (Hangul: 선덕여왕 ] ; 595~610 – 17 February 647) reigned as Queen Regnant of Silla, one of the Three Kingdoms of Korea, from 632 to 647.  She was Silla's twenty-seventh ruler, and its first reigning queen.  She was the second female sovereign in recorded East Asian history and encouraged a renaissance in thought, literature, and the arts in Silla.

Princess Cheonmyeong of Silla: Princess Cheonmyeong (hanja: 天明公主 ) was a princess of Silla, one of the Three Kingdoms of Korea, and is notable as the sister of Queen Seondeok of Silla, who reigned as Silla's twenty-seventh ruler.  She was Queen Seondeok's full sister, her parents being King Jinpyeong and Queen Maya.  Records are unclear as to which of the sisters was the elder, with the "Samguk Sagi" stating that Princess Deokman (later Queen Seondeok) was the elder but with many other sources stating that Princess Cheonmyeong was the elder by a wide margin.

Silla: Silla (57 BC – 935 AD) (Hangul: 신라 ; Hanja: 新羅 ; RR: "Silla " ] ) was a kingdom located in southern and central parts of the Korean Peninsula.  It was one of the Three Kingdoms of Korea, together with Baekje and Goguryeo.  Silla was also one of the world's longest sustained dynasties.  Although it was founded by King Hyeokgeose, the dynasty was ruled by the Gyeongju Kim (김, 金) clan for most of its 992-year history.  It began as a chiefdom in the Samhan confederacies, once allied with China, but Silla eventually conquered the other two kingdoms, Baekje in 660 and Goguryeo in 668.  Thereafter, Later Silla occupied most of the Korean Peninsula, while the northern part re-emerged as Balhae, a successor-state of Goguryeo.  After nearly 1000 years of rule, Silla fragmented into the brief Later Three Kingdoms of Silla, Later Baekje, and Taebong, handing over power to Goryeo in 935.

Queen Maya of Silla: Queen Maya (Hangul: 마야 부인, Hanja: 摩耶夫人) was a member of the Silla royal house and the wife and Queen Consort to King Jinpyeong of Silla, 26th King of Silla.  She was of the royal Kim clan.  She was the mother of Queen Seondeok of Silla.

Later Silla: Later Silla (668–935, Hangul: 후신라 ; Hanja: 後新羅 ; RR: "Hushila " , ] ) or Unified Silla (Hangul: 통일신라 ; Hanja: 統一新羅 , ] ) is the name often applied to the Korean kingdom of Silla, one of the Three Kingdoms of Korea, after it conquered Baekje and Goguryeo in the 7th century, unifying the central and southern regions of the Korean peninsula.  Later Silla was a prosperous and wealthy country, and its metropolitan capital of Seorabeol (modern name Gyeongju) was the fourth-largest city in the world at the time.  During its heyday, the country contested with Balhae, a Goguryeo–Mohe kingdom, to the north for supremacy in the region.  Throughout its existence, Later Silla was plagued by intrigue and political turmoil, mainly by the rebel groups in conquered Baekje and Goguryeo territories, leading to the Later Three Kingdoms period in the late 9th century.

Later Three Kingdoms: The Later Three Kingdoms of Korea (892–936) consisted of Silla, Hubaekje ("Later Baekje") and Hugoguryeo ("Later Goguryeo", it was replaced by Goryeo).  The later two claimed heirs to the earlier Three Kingdoms of Korea, which had been united by Silla.  This period arose out of national unrest during the reign of Queen Jinseong of Silla, and usually refers to the era between the founding of Hubaekje by Gyeon Hwon to the time Goryeo unified the peninsula.

Jinseong of Silla: Jinseong of Silla (c. 865–897) (r. 887–897) was the fifty-first ruler of the Korean kingdom, Silla.  She was also Silla's third and last reigning queen (after Seondeok and Jindeok).  Her reign saw the end of Unified Silla and the beginning of the Later Three Kingdoms period.

Military history of Korea: Korea's military history spans thousands of years, beginning with the ancient nation of Gojoseon and continuing into the present day with the countries of North Korea and South Korea, and is notable for its many successful triumphs over invaders.  Throughout its history, Korea has boasted numerous exceptional leaders who gained outstanding victories against numerically superior enemies.  Famed leaders credited with defending Korea against foreign invasions include: Eulji Mundeok of Goguryeo, who defeated Sui China during the Goguryeo–Sui War; Yeon Gaesomun of Goguryeo, who defeated Emperor Taizong of Tang China during the Goguryeo–Tang War; Gang Gam-chan of Goryeo, who defeated the Khitan Empire during the Goryeo-Khitan War; Choe Yeong and Yi Seong-gye of Goryeo, who defeated the Red Turbans, who later established Ming China, during the Red Turban Invasions; and Yi Sun-shin of Joseon, who defeated the Japanese at sea during the Imjin War.  Other notable leaders include: Gwanggaeto the Great of Goguryeo, who created a great empire in Northeast Asia through conquest, and subjugated the other Korean kingdoms of Baekje, Silla and Gaya to bring about a brief unification of the Three Kingdoms of Korea; Geunchogo of Baekje, who captured Pyongyang and established overseas territories to control much of the Korean peninsula and dominate the seas; Munmu and Kim Yu-sin of Silla, who united the Three Kingdoms of Korea and defeated Tang China to gain complete control of the Korean peninsula; Dae Jo-yeong, who created Balhae from Goguryeo's ashes and reconquered Goguryeo lands lost during the Goguryeo-Tang War; Jang Bogo of Later Silla, who created a maritime empire and commanded a powerful fleet; Wang Geon, who united the Later Three Kingdoms of Korea and established Goryeo as the successor to Goguryeo; and Yun Gwan of Goryeo, who defeated the Jurchens and constructed nine fortresses in Manchuria.

Adalla of Silla: Adalla of Silla (died 184, r. 154–184) was the eighth ruler of Silla, one of the Three Kingdoms of Korea.  He is commonly called Adalla Isageum, "isageum" being the royal title in early Silla.  As a descendent of Silla's founder Hyeokgeose, his surname was Bak.

Supporting paragraphs: Princess Cheonmyeong of Silla Queen Seondeok of Silla
Answer: twenty-seventh