Who is Nicodemus, Deirdre, Cassius from Death Masks and what is their importance?

Nicodemus is a member of the Fallen, a corrupt angel dedicated to evil.

Deirdre is Nicodemus' daughter. She transforms into a demon, armed with razor-edged, snake-like hair.

Cassius is a member of The Order of the Blackened Denarius. He is a human, who's been cursed by the power of one of the Blackened Coins, and he is in service to Nicodemus. Cassius disguises himself as Father Vincent in an effort to enlist Harry's help in obtaining the Shroud of Turin.