Myriad is a technological program created by Astra to "force everyone to her way of thinking", turning them into mindless and obedient drones that could be directly piloted by the user or given verbal commands.
The neural signals from the program could be sent across an entire city through its computing and digital network, though it could not travel through older broadcasting technology. Maxwell Lord found that the signal worked by shutting down the parts of the brain that allow for optimism and hope, the amygdala and the rostral anterior cingulate cortex. At its highest frequency, it could cause headaches that build up into enough pressure inside the head of its subjects to cause it to burst, killing them.
Some aliens where unaffected by Myriad, such as Kara Danvers, J'onn J'onzz, Maxima, the White Martian, and the K'hund, while humans like Maxwell Lord and Cat Grant used "ion blocker" technology, designed to scramble Myriad's signal before it took affect in their minds. Alex Danvers managed to snap out of the program's control on her own will with coaxing from her mother, Eliza Danvers.
In response to the ecological disruption on Krypton, Astra first tried to use it on Argo City, which led to her and Non's incarceration in Fort Rozz (prison). Krypton's High Council deemed the program as too dangerous to be discovered by other worlds, or from the planet's own citizens, due to its potential to upset the balance of power in the galaxy.
At night, though unseen, this program was put into effect on every citizen of National City by Non and Indigo. Every citizen of National City was put under Myriad's control, while Maxwell Lord and Cat Grant were protected by the former's ion blocker technology. Maxwell realized the Kryptonians were using his LTE interface system to broadcast it, which was why they attacked his facility. Superman was also affected by Myriad, which according to Maxwell, was caused by him being raised on Earth and his mind being more human.
Kara, with the help of Maxwell and Cat, managed to broadcast a speech to the people of National City, inspiring hope in them to allow them to break free from Myriad's control.