Elemental Weapon is a Guild Skill, found in the Psijic Order Skill line. The skill basically buffs your Light Attacks and cause them to deal additional Elemental Damage.

Elemental Weapon
2700 Instant Cast Target: Self Base Skill: Imbue Weapon
Infuse your weapon with power, causing your next Light Attack used within 2 seconds to deal an additional 2160 Magic Damage and apply the Burning, Concussion, or Chill elemental status effect. If the power is not consumed in time, you restore 1620 Magicka.

Elemental Weapon is a morph of the skill Imbue Weapon. The other morph is Crushing Weapon. A powerful spammable for Magicka DPS Builds in PVE gameplay.