The du Lac Cross was a cross designed by Josephus du Lac and necessary to decode the rituals and spells written in du Lac's writings such as the du Lac manuscript. Before dying, all crosses were destroyed by du Lac himself, except one that was buried with him. It had the ability to decode any encrypted writing, not just those which he himself had encoded.
Although technically a cross, it did not elicit the same effect from vampires as other crosses did. Not only were vampires not repelled by it, they could actually touch it without suffering from any kind of pain.
The du Lac cross could also be unsheathed to reveal a dagger. The dagger was used in a ritual to restore vampiric health.