Creeping Fear

Question: Are there any Titan Avatars that represent the concept of terror? And if there were, would you place them all in Keku, because darkness kinda represents fear, or would you separate them?

There are none that I'm aware of, although despite my efforts I am not yet a giant magical repository of all mythic knowledge from around the globe, so I could totally be missing someone.

There are very few deities, Titan or otherwise, that could really be said to represent fear or terror. Greece has Phobos and Deimos, the personifications of panic and rout on the battlefield, but on the whole there's almost nobody out there who is specifically a god of fear or a representative of fear. This is mostly because fear, as a general abstract concept, is not something that comes up in very many ancient narratives; rather, gods that are fearsome are usually portrayed as being so because they represent something that causes fear all by itself. People don't fear Nyx because she's a goddess of fear; they fear her because she's a goddess of the deep, impenetrable darkness of night and the ancient powers that formed the world, and those things are scary. People don't fear Mictlantecuhtli because he's a god of fear; they fear him because he's a god of death and decay, and death and decay are terrifying.

This means that 99% of the time, when a culture has a god of a frightening thing, he's frightening because he's associated with that thing, not the other way around. Gods with Darkness, Death, Chaos, Fire and War are often these kinds of scary gods, because they are tied to things that are dangerous or frightening and therefore people tend to give them a wide berth whenever they can. Phobos and Deimos are exceptions to the rule for a few reasons; they're the children of Ares, and as such are meant to represent that he can inspire terror and dread from his incredible warlike prowess, making them more adjuncts to his role than deities in their own right, and they're also from a culture that loves to personify abstract ideas way more than many other mythologies (thus why there are umpty bazillion lesser Theoi who have only a single idea to their name). Outside of the two of them, we don't see the idea much, which isn't surprising - most cultures are not going to worship a god of fear, because they probably don't like fear very much, so most of them won't invent one.

So if we did come up with a Titan with terror as a heavy component, we'd probably put him or her in whatever Titanrealm made most sense for what they do. Terror of dying? Xibalba. Terror of disasters? Whatever elemental realm is most appropriate, or maybe Pakiy. Terror of monsters? Emamu, and so on and so forth. If there were a hypothetical Titan who represented terror itself, as a primal force that was not connected to anything else, we would have trouble choosing the optimal place for them; Keku is certainly an option, since Darkness does represent much of that primordial sense of fear, but it would also depend on whether that Titan did anything else or had any other associations, and exactly how their home culture viewed them.

As always, Scions can totally set out to become gods (or Titans, though we don't recommend it) of an abstract concept like fear, and find ways to knock it out of the park. It doesn't occur much in ancient myth, but that doesn't mean modern myth can't embrace it.