Ynnead is the prophesied Eldar god of the Dead.
Ynnead was a prophesied Eldar Deity who would avenge the Eldar after the fall of their race to the chaos god Slaanesh. Ynnead would be born from the countless souls of the Eldar dead, preserved in the craftworlds' infinity circuits, much in the same way the Emperor was born from the souls of many human shamans. According to prophecy, Ynnead would only be born when the last of the Eldar died, and then fight Slaanesh to avenge them.
Eldrad Ulthran believed that the death of the Eldar race would not be necessary to summon Ynnead. He, alongside a contingent of Harlequins, performed a ritual to summon the god using the crystalized bodies of farseers from every craftworld. However, because the ritual was interrupted by a Deathwatch killteam, they only succeeded in awakening a fragment of Ynnead's power. This fragment found its way into Commoragh, where it chose the dying Yvraine to become its herald and lead all Eldar, whether they be craftworlders, commorites, or harlequins, into a new age. Ynnead, in the form of its avatar called the Yncarne, could now manifest in realspace and take the fight directly to Chaos.
Eldrad and his Harlequin allies planned to summon Ynnead before Abaddon the Despoiler's 13th Black Crusade can destory Cadia, as Eldrad believed that it was the only way to save the Eldar from their impending extinction at the hands of Chaos. The Emperor likewise was aware of the coming birth of the Eldar god, and, through Fyodor, wished to be present for the event in Commorragh.