So after thinking up just what could pose an interesting challenge, I thought of somebody who can take somebody's hard earned work and use it against them better than they can.

Then something like this came to mind, a magic copy ability.

Basically, it works like this. If a creature with this feature is subjected to a magic attack, it can make a saving throw to not absorb the magic it was subjected to. It can choose to fail the saving throw if it wants to absorb it, but it has to exert itself to resist something it does not want to absorb. Once it takes damage by or is subjected to to a magic spell, or magic effect, it can utilize it to it's fullest with no use for spell slots, recharge time, or any restrictions, such as components; verbal, somatic, or material. It's essentially the worst nightmare of Spellcasters and people who rely on effects that are magical in nature, since it can utilize such things far better than the person they took it from, but they can only have one effect saved at a time.

I thought somebody like this could potentially be a humanoid which has training in martial weapons so as to be a great threat in martial and spellcasting categories, meaning no way to approach them is safe. On the other hand, smart usage of spells and limiting your own power can be crucial to overcoming something like this monster.

How does it sound?