The Sleepers will be the source of the components for the production of the new Tech 3 modular ships. But they will also provide something developer CCP feels EVE is lacking - stiff NPC opposition that requires co-ordinated team play.
"More often than not, PVE interactions in EVE are quite limited, merely consisting of watching the NPC explode after a few volleys," said the CCP staffer Ytterbium in the post. "We are not satisfied with that. We want to offer our players true PVE challenge, requiring real team effort and proper gang coordination."
The Sleepers will fulfil that role by being "radically different from the regular pirate factions you are used to", and have attributes comparable to those of players, meaning they "require variations of PVP fits to engage". Much more advanced AI is also promised.
Plenty more on Apocrypha and the EVE universe in general at the EVE Online gamepage.