These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Morlan is the Big Bad of the series and the evil brother of King Aravel. He is either directly or indirectly responsible for virtually every evil that takes place in the series. He doesn’t seem to start out as this, being the Token Evil Teammate of the Council, but he soon drops right off into this territory. He tricks his brother and his allies into beginning an all-out war with the Gorracks by sending his troops disguised as Gorracks into villages and slaughtering the innocent villagers. He regularly takes part in Black Magic, approves of and assists in Cythraul's ceremonies, and makes a pact with Satan himself. He also killed his father and later his brother.
Cythraul, Morlan’s stone-cold, sadistic Torture Technician and right-hand man (or rather, elf), is a cold-blooded killer with absolutely no remorse or mercy in his heart. He is a pure sadist, taking great pleasure in watching others suffer, which mostly takes place in his dreaded Bone Chapel; a temple that is filled with the carefully arranged bones, skulls, and corpses of his victims. He treats his torture of others like an art, knowing every detail in the anatomy of all the races and exploiting them in the most painful way possible. He has no problem with performing his “experiments” on children. He is also a skilled assassin, and tries to kill Abbagael and the baby Telwyn via a poisoned razor on his ring. He even mentally regrets having to kill the baby quickly. This is all just from the first book; he gets evenworse in the next.
Quevara, the Red Queen, is a viciously seductive, Omnicidal Maniac with a fetish for all things red; namely blood and fire. During her reign, she was The Dreaded, and was infamous for slaughtering her subjects just For the Evulz. This included using them as living torches to light her throne room, (often lighting the children on fire first, and forcing their parents to watch before it was their turn.) and butchering her subjects just to see the streams turn red with their blood. When she was merged with the great dragon Raudrim, she took on part of his savage, greedy nature and became bent on destroying and massacring everything in her path.
Moral Event Horizon: Cythraul deliberately invokes this to prove to Ealden that he is beyond redemption. The event in question being when he slowly kills one of his captives and then injects him with a mystical virus that brings the shredded corpse back to life as an undead creature that is non-sentient but obviously suffering. Ealden is horror-struck at this doing and knows that Cythraul has indeed gone too far.
Nightmare Fuel: Cythraul and his Bone Chapel, especially when he starts dabbling in necromancy. He also becomes in-universe nightmare fuel for Abbagael.
The parasitic slime-monsters that can take on the ghostly form of whatever corpse they eat.
Loch being trapped in the wine cellar filled with hundreds of vicious, venomous centipede-like creatures and their Hive Queen.
The Dark Shepards. They are eerie, pale-skinned humanoids in black hooded cloaks with hollow, skull-like noses, thin mouths that are sewn shut with strips of flesh, and eyes that are just small crescent-shaped slits with a pupil-less, pale yellow glow. They are Nigh Invulnerable and can nullify the powers of the Shepards. And they are always silent. Dead silent.
The Untwist: It’s pretty obvious from his initial introduction that Telwyn is the Halfainin. Waiting for the other characters to figure it out is the suspenseful part.