Arcane Energies is a spell that, when revealed by a player entering the dungeon, converts the Physical Attack value of all heroes equipped with weapons into Magic Attack. It also allows the player to draw an additional card from their deck, typically in the hopes of revealing another hero, weapon, light source, etc. to assist in the battle.
This spell is a key tactic in getting around certain monsters with abilities that reduce or prevent Physical Attack damage or require some Magic Attack in order to be fought at all, but can also act as a hinderance if the chosen monster has a resistance or penalty to Magic Attack. This can be partially avoided by simply not equipping certain heroes with a weapon.
Official Glossary entry from 1.4 Rules
You must draw a card when you use this Dungeon Ability.