The Worm Eater was invented by Tsunenaga Tamaki and was given to the Undertakers. After Scar Chain's prison break, Karako smuggled Undertakers' weapons out of the prison and handed them to Ekō Kaidō, in cooperation with Makina. He got the Worm Eater researched by a university lab, planted it in a sword and then smuggled it back in to hand it over to Makina. She used it as a weapon against the Forgeries.
The Worm Eater has been effective against every Branch of Sin, except Ganta's Ganbare Gun and the Wretched Egg's Branch of Sin. It seems that these special Branches of Sin are immune to the Worm Eater.
The only way to get around the Worm Eater, is to attack with the speed of sound, breaking the sound barrier or destroying it with physical force. Known Branches of Sin that can do this are Senji's Invisible Black and Ganta's Supersonic Ganta Gun.