Former Court WizardMeeks, half-brother of Questor Thews, is responsible, directly or indirectly, for every terrible thing that has happened in Landover since the death of the old king. Having persuaded Prince Michel Ard Rhi to abandon the throne, Meeks auctioned off the crown to the highest bidder, disposing of those who backed out and tried to reclaim their money, while Landover, bereft of the kingship disintegrated into anarchy, the country falling apart and the magic that sustained the land dying. When Ben Holiday managed to get the kingdom back in working order, Meeks returned to Landover, with the intent of deposing Ben, undoing the repairs that had been made, and reclaiming his books of magic—books whose power depended on the continued enslavement of the souls of the unicorns trapped within. Willing to consort with demons, slaughter innocent nymphs, and otherwise do everything in his power to ruin Landover, Meeks violated his oaths to the crown and country in every way possible.
Ben Holiday has had many bitter enemies, but none so vicious or persistent as the witchNightshade. Part human, part fairy, and all vindictive bitch, Nightshade marked her first appearance by damning Ben's friends to Hell in Magic Kingdom for Sale/Sold, an experience which nearly killed them all. She made two more attempts on his life in The Black Unicorn and Wizard At Large, and after an enchantment forced her to care about him in Tangle Box, responded by trying to murder his wife and kidnap his newborn daughter. It's in Witches' Brew, however, where Nightshade truly comes into her own. She tries to kill Questor Thews, Abernathy, and the Gnome, Poggwydd, and succeeds in kidnapping Ben's daughter Mistaya. Convincing Mistaya that they are friends, she forces the girl to create a series of monsters which she looses on Ben, intending that he should either be killed, or driven mad by his constant transformations into The Paladin. When this plan too fails, Nightshade gives Mistaya a poisoned brooch and sends her to hug her father, intending that Ben should die at his daughter's hands. Concerned only with her own pride, Nightshade was willing to cross any line if it meant making Ben suffer.
Foe Yay: Ben and Nightshade have this big time, especially in Tangle Box. When they are robbed of their memories and imprisoned in the eponymous box, they have difficulty getting along, although nothing like their normal enmity. Following a bit of Slap-Slap-Kiss, however, they make love. When they wake up in each other's arms, however, the curse coincidentally starts to break, and Ben starts to remember who they are. Nightshade begs him not to tell, fearing correctly that that will finish breaking the curse. Instead, she insists that it doesn't matter who they used to be, and that they should just stay where they are and remain lovers. Ben, of course, decides to break the curse. When they get their memories back and get free, Nightshade is outraged at what had happened and swears, repeatedly, that she will hate Ben forever for what he did. It's an open question whether she is referring to sleeping with her or restoring her memories. It should be noted, however, that, from then on, her hatred for him is expressed by her repeated attempts to kidnap his daughter, who was born right around the time the curse was lifting. This could be interpreted as her subconsciously wishing he had had a child with her instead of with his wife.
Moral Event Horizon: Using the Darkling to compel Willow's mother to dance for him came very close to this for the River Master. Luckily he realized in time and let her go.
Nightshade's scheme in Witches' Brew, which involved trying to have Ben killed by his own daughter.
Nightmare Fuel: An Ardsheal is an ungodly-fast, super-strong, nigh-indestructable humanoid elemental that is virtually invisible when it wants to be. In Witches Brew, the River Master summons one as a bodyguard for Willow and Ben. Then when Nightshade gets a hold of it we find out exactly why Willow is terrified of it.
True Neutral: At first the fairies seem to hew to this trope, but eventually it is revealed that they will take sides, if it means preventing the destruction of Landover or maintaining the Balance Between Good and Evil (or Law and Chaos). And every time they do choose to act, it is always the key to salvation: in book 1 their gift of Io Dust is what allows Ben to get rid of Nightshade and stop Strabo's depredations; in book 2 Dirk's assistance to Ben and the gradual alteration of Willow's dreams allows them to stop Meeks and save the unicorns; in book 4 it is the fairies allowing Willow and Ben to encounter each other in the mists, circumventing the Tangle Box, that helps him recover his memories of his true self (and them sending Dirk to Willow allows her to gather all the soils she needs and give birth to Mistaya); and in the most recent book they send Dirk again, this time to Mistaya to help her save Libiris (and Landover) from Crabbit and the demons.