The Old Ones once created the race of Eldar, who they may not have anticipated taking their place. Nevertheless, the Eldar controlled an empire truly equal to their Necron foes before their fall, and with your new knowledge this glory may one day be seen again. You gain the knowledge of all of the technology & technique (psychic or otherwise), that once allowed the Eldar mastery of the galaxy.
Craft psi-implants that augment or negate psychic powers, make your soul truly immortal, create artificial gods, manipulate flesh to grow or modify entire sapient species, weaponize the Warp masterfully, sing psychoreactive materials like Wraithbone into existence, build whole new Craftworlds, create pocket-dimensions, move stars across the galaxy using Warp-gates, build Wraithbone Automata, & even construct a new Webway from scratch.