A revenant is essentially a zombie, or a reanimated corpse. It emanates a magical fear so powerful it can scare someone into a state similar to catatonia and cause their hair to turn completely white.
Rise of the Evening Star
The revenant was a guard over the vault where the magical artifact was hidden. He radiated powerful magical fear and turned victims albino and put them into a catatonic state. To defeat the revenant, Seth had to pull the corrupted nail out of it's neck, which killed the Revenant completely and released all those who'd fallen victim to it from their catatonic states. The corrupted nail later proved to be a dark talisman which gave Seth immunity to magical fear. After the Sphinx had the Sorensons remove a mysterious stranger from the Quiet Box who was imprisoned before the Sorensons were even caretakers of Fablehaven, he brought the talisman to Kurisock who used it to create the Shadow Plague.
The nail was destroyed when Lena Burgess touched it with a stone imbued with the power of the Fairy Queen's shrine at the cost of her life.