The «Guilty Thorn» (ギルティソーン, Giruti Sōn?) is a player-made spear created by Grimlock.
Chronology
Aincrad Arc
A Murder Case in the Area
Guilty Thorn is the murder weapon used to supposedly kill Caynz within the Area, which was practically impossible. The spear was brought to Agil for appraisal, who discovered that its creator was Grimlock. However, it was later proven that the murder was faked and that the spear was only used to destroy Caynz's armour.