The Umbra is a Realm made of the lingering dreams of a dead god. The last of the Excubiae born from Z'zhuul's initial storm of creation, Puer Hostiæ came much later than the others. He was seen as a neonate, and never developed the higher processes his siblings possessed - or was possibly just created without that capacity, as the other Excubiae never had to mature to reach Their full facility. In fact, when Jehovah and Lilith created the human race, though They did so in Their own image, They designed the human infant to resemble Puer Hostiæ as well, in tribute to their youngest brother.
From the time He was conceived, His presence caused a calming effect and near-overwhelming feelings of love to all in His presence - including most of the other Excubiae. It's believed the creation of the Umbra was an accident, or an errant thought. Further, it's believed that everything He created after that moment was an accident as well - He never seemed quite able to focus or control his creative ability, and had no interest in controlling His manifestations after they were brought into being. As a result, his Realm came to be known as "The Playground" - it was a representation of pure creation, devoid of any planned destruction. This caused anarchy, due to the many varied creations living side by side in dissimilar, even contradictory environments. But the chaos was left for the environs to work out, without the intercession of their creator.
His crowning achievement was the creation of the chimera - an autonomous changeling that impressed His siblings, with the exclusion of Carnifice, who had always been immune to Puer Hostiæ's euphoric effect on the others. Enraged that his childish brother had bested his own crowning achievement, the event culminated in the infant god's death. Carnifice remorselessly slew His baby brother, in front of all the other Excubiae, who had no idea it was even possible for one of Them to die. Then, Carnifice shut Himself off from the others and left Them to Their grief.
Now the Umbra has come to be a shadowy reflection of itself. The dreamy environment have taken a much darker tone than when He was alive. The Umbra has become darker and corrupt in nature. Some areas of the Realm are pleasant, reminiscent of paradise. Some are horrible and nightmarish. But there is an undercurrent of malevolence within it all. The most beautiful faces are scarred, the most impressive structures are flawed, the most altruistic-seeming acts are secretly self-serving in nature. Every being and every situation presented within this Realm are infused with a form of manipulation. The other Excubiae leave the Umbra alone. Some see its continued existence as a fitting memorial to the deceased child. Some see it as merely unworthy of consideration. Indeed, even the followers of the Great Nameless seem to cut the Umbra a wide berth, but none truly know if it's because there is something about it that gives them pause, or if it's because they consider it beneath their attention.
But something stirs in the Umbra now. Something different. Something... angry.