Sigil -1 can be gained from LEGO cheat
When typing the LEGO cheat code in Strife for the sixth time in a row, the player will change to an invalid Sigil type which would be internally numbered -1 (the normal five forms of the sigil being numbered by the cheat code as 1 through 5, with zero meaning "no sigil").
When this abnormal form of the Sigil is raised by the player, a light flash will occur as if a weapon is being fired. When the player attempts to utilize this strange weapon, a corrupt sequence of states will be displayed which pull various graphics from the Sigil itself and from an unrelated weapon, the grenade launcher.
There are no known ill consequences from possession or use of Sigil -1, and it has absolutely no offensive firepower whatsoever - no projectile is emitted by the weapon because none of its frames actually call an attack action function. Firing it additionally does not damage the player as normal use of the Sigil ordinarily would.