The monumental altarpiece with the Martyrdom of St. Agnes, commissioned by Pietro Martire Carli for the convent of Sant'Agnese in Bologna, was begun by Domenichino in 1619 and completed in Rome, where the artist had sojourned in 1621.

In this complex painting, the artist established himself as one of the most important representatives of the seventeenth-century classical ideal, beautifully expressed in the works he painted between Bologna, Rome and Naples.

The composition is set out on two levels, the earthly one and the celestial one, according to the traditional scheme. On the upper level, bathed in bright light, is the Trinity, surrounded by a choir of graceful musician angels.

The six different instruments played by angels, all illustrated in their minute details, bear witness to Domenichino’s great passion for music.

On the lower level is the martyrdom of the Saint, observed by some soldiers and women. In the background, various figures look on from the top of a loggia, between whose columns we can see the countryside in the distance.

The very young martyr is depicted as she is dying, pierced in the throat on the orders of Aspasio, vicar of the prefect, whose son had been rejected by Agnes, who had already devoted herself to chastity.

Domenichino, although portraying a violent scene of torture, avoids any excessively bloody tone and with his classical style, softens the turbulence of the drama: the idea of martyrdom is expressed by emphasizing the difference between the victim’s figure and the executioner sinking the dagger into the saint’s throat, while he forcefully pulls her long hair with his left hand.

The saint, on the other hand, with her ecstatic gaze, is already engulfed in the vision of paradise and the promise of eternal salvation, sealed by the presence of the angel who is receiving the palm of martyrdom from Christ's hands.

In the foreground, next to the stake, which the Saint had miraculously survived, the bodies of two men, probably identifiable with two executioners, lie on the ground.