Melf's acid arrow
From Ardrana
By means of the Melf's acid arrow spell, the wizard creates a magical arrow that speeds to its target as if fired from the bow of a fighter. The arrow inflicts acid damage on its target, with the corrosion lasting longer as the wizard grows more powerful.
Note: This version of Melf's acid arrow originally appeared in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.