The Pool of Nemhain is the last of the five Gateways that separates the world of the living from the world of the dead (the Spirit world). The waters of this lake are as black as night, and as still as death itself, presumably because it is a gateway to the dead.
The Dochraid told Morgana that there were five Gateways to the Spirit world. One of them could also be the Lake of Avalon, that connects the living world to Avalon. The Cailleach could also be the gatekeeper to the Pool of Nemhain, since it is just a portal to the Spirit world.