Dormin says the following upon retrieving the item:
That is the "Harpoon of Thunder"... With it, thou shalt be able to inflict greater damage upon a colossus from afar...
The damage done by a fully charged throw is about the same damage as a full stab with the Ancient Sword or the Sword of the Sun. Even though it deals serious damage, the Harpoon is still a ranged weapon and thus cannot kill a colossus. A colossus's life bar can be almost depleted with the use of the Harpoon, but in the end a stab from a sword is still required to kill it. It will, however, like the swords and Flash Arrows, trigger minor sigils with a succesful hit.
- The reticle seen while aiming the Harpoon of Thunder is different than that of the Bow & Arrow, in both color and shape. It is green and more circular.