The oldest named Eldar in the fluff, millions of years old and present for the War in Heaven - a Shadowseer.
Some relevant fluff about her:
The last Phaeron of the dynasty— Ahmontekh, called the Crimson Scythe—was a general of great repute. It was Ahmontekh who, according to the Lamentations of Yr, defeated the proud Nuada, beloved of the Old Ones, and who plunged his mighty war-scythe into the hallowed ground of Xoth. As the War in Heaven neared its climax, it was Ahmontekh that breached the Walls of Ib and who was first to pass through the Dolmen Gate on the brink of the Ebon Void. It is said that Ahmontekh defeated beings the ancient Eldar revered as gods, and that even to this day the last of the Old Ones’ progeny hold him in a unique blend of hatred and fear. Yet, of all the legends told of mighty Ahmontekh, none are repeated so often as that of his betrayal at the hand of his cousin Setii of the Charnovokh Dynasty.
Though none know it, Ailill Nuada is in fact the most active of all of the members of the Conclave of Tears. She travels extensively through the dead systems of the Outer Reach, seeking to piece together the separate elements of what she knows to be a dark fate slowly creeping across the region. Alone of all her peers, Nuada recalls the face of that which slumbers beneath the surface of the dead worlds of the region, for she looked upon it when last it stirred. She knows what is soon to awaken, and she knows the steps that have been taken to ensure the powers that hold sway in the galaxy remain in balance, though each will suffer undreamed of destruction in the process.