A fidget sinner also known as a devil circle is a toy designed to spawn demons and murder angels.
It is a common device commonly used in The Nether (Minecraft) and The Underworld (Terraria), but due to large interest it was exported to many other dimensions in many other games.
Fidget sinners were originally made in The Nether with the goal to assassinate Notch, the creator of Minecraft. Some fidget sinners were found in The Heaven Dimension (Minecraft). They were most likely hidden there by corrupted angels that sold their soul to Herobrine. In Terraria, fidget sinners were made by combining lava and moon stone.
In some versions of Terraria the Moon Lord can be seen playing with a fidget sinner right before his boss battle. If you don't manage to defeat him your character along with your save file will be deleted permanently.