The Mausoleum of Shams al-Din Iltutmish (1211–1236) is located in the northwest corner of the Qutb complex, next to Iltutmish’s own extensions to the Qutb Mosque.

Shams ad-Din Iltutmish (in Persian شمس الدین ایلتتمش; died April 30, 1236, ruled 1211–1236) was the third of the Mamluk rulers who governed the former Ghurid territories in northern India. He was the first Muslim sovereign to rule from Delhi and is considered the actual founder of the Delhi Sultanate. This is the tomb of Shams ud-Din Iltutmish (reign: 1211–1236), who was the third ruler of the Delhi Sultanate, belonging to the Mamluk dynasty. Shams ud-Din established a new nobility, the council of forty, known as Turkan-e-Chihalgani. In 1236, Iltutmish died and was buried in the Qutb complex in Mehrauli.

Iltutmish literally means "protector of the kingdom" in Turkic. Due to the absence of vowels in historical Persian manuscripts, various 19th–20th century writers read Iltutmish’s name differently: "Altamish," "Altamsh," "Iyaltimish," and "Iletmish." However, the Tajul-Ma'asir manuscript from 1425–1426 shows the vowel "u" in the Sultan’s name, confirming that "Iltutmish" is the correct reading of the name.

Sold into slavery at a young age, Iltutmish spent his early years in Bukhara and Ghazni under various masters. In the late 1190s, the Ghurid slave-commander Qutb ud-Din Aibak bought him in Delhi, making him a slave of a slave. Iltutmish rose to prominence in Aibak’s service and was granted an important iqta (feudal estate) of Badaun. His military campaigns against the Khokhar rebels in 1205–1206 attracted the attention of the Ghurid ruler Muhammad of Ghor, who freed him even before his master Aibak was freed.

After Muhammad of Ghor’s assassination in 1206, Aibak became the de facto independent ruler of the Ghurid territories in India, with his residence in Lahore. After Aibak’s death, Iltutmish overthrew his unpopular successor Aram Shah in 1211 and established his capital in Delhi. He then consolidated his power by subduing several dissidents and fighting other former Ghurid slaves such as Taj al-Din Yildiz and Nasir ad-Din Qabacha. Between 1225 and 1227, he subdued former subordinates of Aibak who had established an independent kingdom centered in Lakhnauti in eastern India.

In the early 1220s, Iltutmish mostly stayed away from the Indus Valley region, which was engulfed in conflicts between Qabacha, the Khwarezmshah dynasty, and the Mongols. In 1228, he invaded the Indus Valley region, defeated Qabacha, and annexed large parts of Punjab and Sindh to his empire. Subsequently, the Abbasid Caliph al-Mustansir recognized his authority in India. In the following years, Iltutmish suppressed a rebellion in Bengal, captured Gwalior, plundered the cities of Bhilsa and Ujjain in central India controlled by the Paramaras, and expelled Khwarezmian subjects in the northwest.

He had to defend his empire against Mongol invasions and also deal with Rajputs, conquering Rajasthan and Gujarat. He initiated a rapid process of Islamization in Punjab and Haryana. Iltutmish halted the Mongol advance in 1221 led by Genghis Khan. He completed the construction of the Qutb Minar, started by Qutb ud-Din Aibak. He also built the Quwwat-ul-Islam Mosque in Delhi. Iltutmish died in 1236. He did not consider his sons worthy rulers, so his daughter Razia Sultan (also called Radiya or Raziyya) became his heir.

The mausoleum is built of sandstone and has a square shape, measuring 9.1 meters on each side and 8.41 meters high up to the base of the (presumed) dome. It was constructed from new material, without using spolia employed in other buildings of the Qutb complex. It has three entrances—on the north, east, and south sides. The western wall, facing Mecca, contains a mihrab in the central niche of three. The upper chamber, now open to the sky, contains a richly decorated marble cenotaph. On the northern side, there are steps leading down to the burial chamber.

The square base of the tomb transitions into an octagon above the head by means of arches shaped like pointed domes. These arches are constructed using corbelled slabs rather than true arches. The structure lacks a dome or any other roof. It is assumed that the original dome collapsed; it is also noted that the tomb’s outer walls are relatively thin (2.2 meters) and might not have withstood the lateral pressure exerted by a dome.

The entrance and parts of the outer walls are faced with quartzite, and the gates are decorated with carvings; however, the true decorative richness of the tomb lies inside. Intricate epigraphy on the walls, arabesques, and geometric patterns adorn the western niches and the mihrab. Inscription panels include Kufic, Tugra, and Naskh scripts illustrating chapters of the Quran. The sandstone carvings also feature Hindu motifs such as lotus flowers and bells on chains.

Sources:

https://en.wikipedia.org/wiki/Qutb_Minar_complex

https://en.wikipedia.org/wiki/Iltutmish

https://simple.wikipedia.org/wiki/Shams-ud-din_Iltutmish