Wanda was possessed by Nightmare during Multiverse of Madness. (TVA 2024 #4)

In the recent MCU tie in comic, TVA (2024) #4, it was revealed that whatever form of Wanda or the Scarlet Witch that was imprisoned in the TVA underground department the issue before, had been Nighmare the entire time in disguise.

Even though a good number of people deny this variant to be the MCU/Sacred Timeline Wanda, let's just considering her the main one for this theory sake. Now, we weren't shown the direct aftermath of Wanda using the Darkhold at the end of WandaVision looking for her children, only that by the time of DS2 she had been completely corrupted by the Book of the Damned and began to disrupt space-time norms to achieve her goals.

Incursions, the overarching plot point which arises when a person travels to another Universe from their native one, is the main and biggest threat that posed Wanda's plan, in which the collision of universes could destroy one or both of them. We even saw a Incursion Universe, of Sinister Strange, in the film itself.

Here's my theory. Nightmare has been described in the comics as the embodiment of the collective dreams of all living things in the Multiverse, an Abstract Entity, and an equal of Eternity, who is the Multiverse itself taken a form. It wouldn't be wrong to categorize them as opposing forces, in which one represents an ordered reality, and the other represents a disordered reality (Nightmare). So, when a Universe dies, its energy goes to Nightmare, who, feeding off that energy becomes stronger. Incursions here, are a mechanism for the entity to gain more power, through the collective trauma of the inhabitants of the dead Universes. This plan of setting the Incursions into motion was carried out by Wanda, whom the Abstract Demon took over, to carry out his scheme. Essentially, Wanda is an Avatar for the higher Dimensional entity to interact with normal Universes and put his plan in motion.