Nightmare Fuel / Magical Record Lyrical Nanoha Force

• Signum's defeat by Cypha. The result... well, is not pretty.
• Lily's flashback when she becomes a guinea pig for some Eclipse Virus experiments, prior to her meeting with Thoma and friends. This includes witnessing people die upon touching her.
• Thoma's time on the Huckebein Warship. He ends up learning all the horrors of the Eclipse from the Fortis and is now aware of what he will eventually turn into, is caught in the crossfire between the Huckebein and Section Six and triggers the Divide Zero, and his Eclipse symptoms take a turn for the worst. So what does he decide to do? He comes to conclusion that he has become a monster and opts that he would end his life instead of taking lives. He says this in front of Subaru before leaping off the ship. Keep in mind it is implied Thoma is only two years older than Vivio ^note Actually three years older. Note that ViVid set two years before Force, and Vivio is ten years old. So that means Thoma is 13 years old, by the time of ViVid(and one years older than Einhart, if you counting her cameo in Chapter 17).
• In Chapter 29, we learned that Hades Vandein is the Not-So-Harmless Villain. What did he do next after he freed himself from Curren's spell? He beat the shit out an exhausted Veyron and took Vey's heart while it was still beating. All with a cheerful smile on his face before doing it.
• The Eclipse Virus in general. This is one of the biggest reasons why this series so dark. When a mage or ordinary human being comes in contact with Eclipse infected person, they become an Eclipse Driver. The infected is granted with high level regeneration, can switch vision into an HUD-like manner to see everything easily, and of course, viralizing. The side effect however, is that the infected person slowly but surely becomes something inhuman, either by altering or taking away the infected personality. Also, the infected become more hungry and thirsty for blood. If the infected person cannot adapt to the Eclipse Virus or enjoys the power a little too much, they become nothing but a lump of flesh. This was even an fear for Subaru when she found out that Thoma is infected with this very same virus and was concerned at the possibility he would end up like the Hückebein. This is arguably the most disturbing thing in entire Nanoha franchise.