Which good fairy modifies the curse so Aurora will not die but fall asleep?

Merryweather is the good fairy who modifies Maleficent’s curse so Aurora will fall asleep instead of die. She cannot erase the curse completely, but she changes its consequence and preserves a way for Aurora to awaken.

At Aurora’s christening, Maleficent announces that the princess will prick her finger on a spinning wheel’s spindle before sunset on her sixteenth birthday and die. Merryweather’s own magical gift is interrupted before she can present it, but she uses her power after Maleficent leaves.

Merryweather explains that Aurora will not die when the curse is fulfilled. Instead, she will fall into a deep sleep and awaken when her true love gives her a kiss. This change drives the film’s final act: Prince Phillip must escape Maleficent, defeat her dragon form, and reach Aurora.

Flora and Fauna are also essential guardians, but they do not alter the curse. Flora gives Aurora beauty, while Fauna gives her song. Merryweather is therefore the specific fairy associated with softening the curse, not Maleficent, who created it, or the other two good fairies.