An item where you sacrifice health, etc., for more power is an archetype. Akin to someone selling their soul for rock 'n' roll. It could be a decent addition. However, allowing it to just double damage via the Void element sword seems too easy. It distracts slightly from the rest of the design, and especially from the opponent - who should be an important source of pressure here, not a punching-bag. The item could stand to focus on one or two notable aspect, so that the full pathos and significance of these can be brought out. Void weapons at present tend to be more careful and integrated into the 'quests,' so this might not belong with this rare element just to inflict free damage.
What might be interesting is finding such items in Death's Domain - if a player went there.
A zard which is inverted to be helpful could be slightly amusing, at least. It might defy expectations. However, if too frequent it would get in the way of the atmosphere around zard-quests. You suppose it could lead to some sort of parody of the Dracomancer saga. If zards were made more prominent in higher level quests as well, then having benign zards which fade away over time would be a useful shift. It would make them seem slightly more hostile later on, which could be good to avoid them being exhausted quickly.
From beyond my past, from beyond your grave,
you are, you are.
Your memories are mine, they chase me through all time,
and you are, you are.