Credit goes to MathewMii, Special thanks JamesG for the use of Poochpamp.

Out of all the knights in Siwa, None were stronger than the Lugia captain who was in command of ten elite soldiers wielding elemental weapons, The Lugia Captain used an Aqua Spear. Upon his defeat he was taken to an undersea shrine with his spear that only those in Monas family can break. Mona the 2nd saw alot of her favorite captain in Mercury and still missed him, but with the war on Kroko coming close, Mona escorted Mercury to the undersea shrine where she undid the seal and Mercury had to prove his worth.
Unlike the other weapons, this one was wielded by proud knight that only pure noble Lugias can wield, and Mercury became a Noble Lugia when hired by the royal family of Pangorio. Mercury challenged the Lugia Captain's spirit and Mercury came out on top, even the captain was amazed that Mercury was more stronger than other Lugias, he just had to undergo one more trial, the shadow and evil in him has to be purged. When Mercury touched the spear, it detected some evil in his heart, so the spear asked Mercury if it was ok.

Without any hesitation, any evil or malicious intent in Mercury was purged and he was now able to wield the Aqua Spear, which shed a tear in Mona the 2nd since she was seeing her Captain again. The Captain amazed at Mercury felt proud to hand his spear over to Mercury and promised to alway safeguard the family, which he agreed on. Mercury now had to master it, and since the Lugia Captain gave it to Mercury, he's the only one who can wield its divine might. Like many other weapons it comes with elemental benefits and curses if anyone other than its master touches it.

End

This Spear like the lance can't really cut, its used to amplify a Lugia's Psychic and Water powers. Its one special ability is that anyone whose hit with the blade has the evil and shadow in them purged of all evil, but it only works on those with artifical evil, those who are pure evil won't have any effect, it purifies the body, but not the emotions. Like Crescent, Mercury is always seen wielding it and even the Monas watch Mercury train with it. Mercury does like showing it off, but that Weedle doesn't seem impressed, but the look on Kimos face shows he wants Mercury to stuff the Weedle. Since Mercury's the only one who can wield this spear, he's the only one that can break the Lugia Captain's seal on Queen Ayas tomb for Siwas other divine treasure. The Silver Wing that's on the spear belonged to the Lugia Captain himself, Mercury wanted to replace it with his, but felt it wasn't ready yet. The Spear helps Mercury alot on Mercenary jobs, and Kimo mostly touches since it turns him into a water resistant diaper.
Like many other weapons, if anyone whose not its master touches the weapon and they succumb to its curses, in the spear's case, they get turned into either a disposable diaper, Cloth or Swim Diaper. Even Venus and Lavender aren't allowed to touch it. If other Lugias try to touch it, it'll just be massively heavy for them to lift.

Mercury belongs to Me.
Poochpamp belongs to JamesG
Lugia and Weedle belongs to Game Freak.