The Barrier is a weapon never seen in the final version. It is a staff that has bat wings that uncurl when the player takes it out. When fired, the player will move the Barrier around, but nothing happens. Based on the weapon's name, it is supposed to create some sort of barrier.
Of note is that the barrier's .dll file is named “Celestril”, which is a character shown in Daikatana concept art, but never appeared in the game. It is likely that the player was supposed to obtain the Barrier from Celestril in one way or another.