Demon Night and Thorny Grave are two attack Psynergies available to the Samurai class series, thus only Venus and Mars Adepts have access to them. Both Psynergies are Venus based, have a chance to haunt a foe, and have a range of three.
Demon Night: "Strike with a myriad of monsters": Demon Night is available to an Adept who is a member of the Samurai class. It is learned at level 18 and costs 12 PP and has a base attack of 60.
Thorny Grave: "Summon a loathsome fiend": is available when an Adept becomes a member of the Ronin class, and is therefore unavailable in Golden Sun. It replaces Demon Night and increases the base damage to 170. The PP required also doubles, meaning that the Psynergy costs 24 PP.
Overall, these two abilities are not as impressive as others available to the samurai class. If you're looking for a venus psynergy to use in this class, it might be better to use helm/skull splitter. The main situation where thorny grave becomes more useful is when you're fighting multiple ememies with low Venus resistance. Even then skull splitter and quick strike are generally more useful. As for the haunting side effect, an enemy using this move will get much more benefit from this skills effect than if the player were to cast it.