What are waves and how do they work?
Technically known as Waves of Misfortune , they are a seemingly random but frequent phenomenon in which the sky turns red and hordes of monsters enter through portals.
A wave, once started, will continue until the planet is complete or the overriding monsters have been repelled or defeated. A wave will mainly be concentrated in a specific area around the world, where the legendary heroes, whose goal is to beat the waves, will be summoned with their companions.
If the primordial monsters of the Wave have a chance of being defeated, the portals will close and the sky will return to normal, suggesting the end of the Wave.
As will become clear in The Rising of the Shield Hero Season 2 , there are also different types of waves. The 4 Guardian Beasts, including the Spirit Turtle, technically trigger their own waves, but these are subtly different and not meant to involve legendary heroes.
What causes waves in The Rising of the Shield Hero ?
Although not explicitly mentioned in the anime , the waves are caused by different worlds each with different heroes crashing into each other. Waves occur as soon as 2 universes come into contact, causing their borders to bleed and monsters to invade.
Once 2 universes collide, both experience the wave. If one manages to repel the wave but the other world fails, the lost planet is finished and the victor regains power. This detail explains the activities of the heroes of the other world in season 1 of the anime; Once Glass crashes on a wave and the heroes of the other world try to eliminate Naofumi.
Furthermore, it is later born that the frequency of the waves is being manipulated by a popular mysterious entity like the Devourer of Universes, who seeks the power emanating from the waves and the colliding universes.