The first Sultan who required and obtained letters of investiture from the Caliph (Khalifa) was Iltutmish.

Iltutmish was the third Sultan of the Delhi Sultanate and ruled from 1211 to 1236. He was the first Sultan to establish a strong central government in Delhi, and his reign marked the consolidation of the Sultanate's power.

Letters of Investiture from the Caliph (Khalifa)
In medieval Islamic political tradition, the Caliph, who was considered the successor of Prophet Muhammad, held a position of supreme authority and was the spiritual leader of the Muslim community. The Caliph had the power to grant legitimacy and authority to rulers within the Islamic world.

Reasons for Iltutmish Obtaining Letters of Investiture
- Consolidation of Authority: Iltutmish sought the letters of investiture from the Caliph to strengthen his legitimacy as the ruler of the Delhi Sultanate. By obtaining the Caliph's approval, he could establish his authority over the vast territories under his control.
- Religious Legitimacy: The letters of investiture provided Iltutmish with religious legitimacy. As a devout Muslim, he wanted to ensure that his rule was recognized by the highest religious authority in the Islamic world.
- Political Recognition: By obtaining the letters of investiture, Iltutmish gained recognition from other Muslim rulers and states. It established his position as a legitimate ruler and allowed him to engage in diplomatic relations with other powers.
- Symbolic Importance: The letters of investiture served as a symbol of power and authority. They were displayed publicly, emphasizing Iltutmish's status as the chosen ruler by the Caliph.
- Succession Planning: By obtaining the letters of investiture, Iltutmish also secured the succession for his descendants. The recognition from the Caliph ensured that his successors would also be considered legitimate rulers.

Significance of Iltutmish's Action
Iltutmish's action of obtaining letters of investiture from the Caliph set a precedent for future rulers of the Delhi Sultanate. It established the practice of seeking religious legitimacy and political recognition from the Caliphate, which continued throughout the Sultanate's history. This practice further strengthened the position of the Delhi Sultanate as an Islamic state and allowed its rulers to assert their authority over the vast territories they controlled.

In conclusion, Iltutmish was the first Sultan to require and obtain letters of investiture from the Caliph, establishing his religious legitimacy, political recognition, and securing the succession for his descendants.