Aura Siphon

You siphon some of another creature’s magical aura, imbuing you with extra strength and power at their expense. If a target is in range and you have witnessed them use magic, you can use your action to steal a percentage of their aura for the next minute. The target must make a constitution saving throw against your spell save DC. On a fail, roll a percentile die. The target loses a percentage of spell slots (or magical abilities) equal to the percentage you rolled, and you gain access to those spell slots for the duration (i.e. on a 50, you steal half of the target's spell slots). The effect ends if you fall unconscious or drop to zero hit points before the spell expires.