This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Wayfarer Redemption
The Demons in general, but especially Qeteb. The 2012 mania. Images of the Qeteb baby and StarLaughter trying to make it drink her milk even when it is dead. If you make it far enough, you then get struck by images of Qeteb-man and Niah-corpse having sex and then Qeteb's body stretching to fit his demonic black armor.
Borneheld's demise. A hero did that to bring back his cousin. How touching. Though to be fair, Axis was revolted by the idea.
The Demonic Hours and just what they do. You'll be jumping at fog for days.
Timozel, and the sheer extend to which he was manipulated and brainwashed. For those who actually liked his character and his devotion to Faraday, that...that was just sickening.
The rapes of WolfStar. It almost reached Urotsukidouji level gruesome.
Faraday's...explosive...reaction to the scepter when Drago leaves Tencendor at the behest of the Timekeepers.
Let us not discuss the on demand Body Horror provided by the Time Keepers. Spontaneously growing udders to defile a lake. Making tentacles to rape people. These things are just sick and wrong.
The attack on Carlon. The fact that the rabid, insane animals only were attacking children. By gorging out their stomachs and crawling inside. Ick.
Axis' Salutory Lesson.
Gorgrael is a frightening looking beast that "takes Caelum" at DragonStar's request, scarring the boy for life. All for inheritance. Oh, and DragonStar? He is a baby.
Speaking of this baby, it had to have its magic taken away for everyone's safety. Let me reiterate. The parents are Physical Gods. If a baby scared them that much...
Axis watching his own birth by using the Song of Recall.
Zenith's rape by Wolfstar and falling pregnant, losing her body to Niah, who is her grandmother and then forcing Niah out by pushing her soul into the undeveloped baby (with Faraday's assistance), forcefully aborting the fetus and throwing it against the wall to crush it's skull. Spoilered due to how disturbing it is.
Zenith's death, when she's eaten alive by the Hawkchildren until all that's left is her head and her arm, just because StarLaughter decided she got in the way of her and WolfStar.
Axis attempting to kill Azhure at the end of Enchanter due to believing that she's WolfStar, then watching and even experiencing Azhure's Dark and Troubled Past.
Fridge Horror given the speed the kids develop in Icarii society - Caelum is very, very aware, lucid, capable of mental speech and such...and is a baby. Mostly unable to move. Prone to bodily dysfunction. And then there's how his Enfant Terrible brother sold him up the river to Gorgrael, and how utterly defenceless Caelum was in the clutches of a being raised to be hateful, evil and sadistic. No wonder Caelum later on grew up rather dysfunctional and messed up.
And on said brother's end, being De-Power'd and ending up seeing your family live forever and have magic, while you have nothing, everyone hates you for a reason you don't even remember, and people try to keep you disenfranchised for no better reason than "Gods Say So". The banal existence that Drago lives in at the start of his trilogy is probably only the half of it.