Unaware of the dark presence hidden deep within her, Aegwynn gathered Sargeras's colossal, broken form to seal it away where it could harm no one else. She considered many locations to serve as the demon lord's final resting place. In the end she chose the ancient night elven city of Suramar, some of which had been blasted to the bottom of the sea during the Sundering.
During the War of the Ancients, the Legion had attempted to open a gateway within Suramar itself. This plan was foiled by a sect of Highborne led by Grand Magistrix Elisande. These powerful sorcerers created a series of enchanted seals to close the demons' portal and also negate nearby fel energies. When the Sundering later tore through the world, the part of Suramar containing the Legion's failed gateway was sucked beneath the waves.
It was these lost ruins that drew Aegwynn's attention. Knowing that the Highborne's seals would nullify whatever evil still lingered in Sargeras's avatar, she buried the demon lord's broken body within the sunken portion of Suramar. Aegwynn hoped Sargeras's remains would like undisturbed there until the end of time.