Mysterious Cubes

The Mysterious Cubes are a series of cuboid objects that are used to communicate with God.

Overview
As explained by Blast, the cubes act as a transmitter that distort space and allow for communication with God. Anyone who comes in contact with a cube is able to hear the voice of God in their head.

One of the cubes was first seen 18 years ago during a flashback of Tatsumaki when she was saved by Blast from a monster. Another cube was also found by Saitama, Manako, and Flashy Flash in the rubble of the Monster Association Headquarters. Blast stated that there are multiple cubes scattered around the world, and that he has been collecting them as a hobby.

The cubes appear to be extremely heavy, as one was able to get Flashy Flash's arm stuck, and sunk into solid rock after being dropped by Saitama. Additionally, Manako was unable to lift it back up.

Monster Association Arc
One of the cubes appears for the first time during Tatsumaki's flashback as she recalls the events of 18 years ago when she was saved by Blast from a monster. Here, it is revealed that Blast was holding a black cube when having a conversation with Tatsumaki at the time. Later, when Saitama and Manako are helping free Flashy Flash, they come across another cube which had pinned Flashy Flash's arm down. Saitama inspects it before throwing it away without having any idea what it actually is. As Tatsumaki begins to lift the remains of Monster Association Headquarters into the air, Saitama along with Flashy Flash and Manako peer out into the hole expecting to see the underground, only to instead be greeted by a black void, with a mysterious figure sitting prone inside, though the three cannot see it.

God's voice begins to speak in their heads, saying that if they touch the cube and make a wish, the being will either grant them power, or they will "be confiscated." Immediately afterward, Blast appears and is relieved to find that the group did not accept God's words. He takes the cube, transferring it through a portal to an unknown location.