Magical Aura is a spell from the base set.
It converts the weight of all weapons to 0, and allows you to draw a card. As a Dungeon: ability, it converts the weight of all weapons currently revealed - any weapons revealed after Magical Aura's ability is used (say, a weapon drawn as the draw one card) will not have their weight revealed.
Official Glossary entry from 1.4 Rules[]
When played with a Polearm, the Hero must still have a Strength of 8 or more to gain the +6 bonus. You must draw a card when using this Dungeon Effect.