I - Rules
(see WOD, p.6)
The overlord's cost to play trap cards is reduced by one threat for each rank of Trickster a figure has, while it remains in play. If more than one figure in play has Trickster, the effects are cumulative.
II - References
Heroes with the Trickster ability: none.
Monsters with the Trickster ability: Kobolds (master only).