I feel a bit iffy about this power up. Is it permanent or temporary? Because, the core principle behind a celestial mage is to rely on the spirits and work with them in a battle. By doing this, Lucy broke a taboo in a sense: she broke the key, absorbing Aquarius magic into herself and becoming able to use it. Not exactly sure I enjoy this plot line. Though it may be a temporary thing allowed by the king of spirits.
King of Hades is really really powerful. A bit too much so for even the spirit king. I don't see the spirit king winning.
All in all, its I think the first attempt that Hiro has made of provoking a real plot twist with unforeseeable results and a certain negative plot line (Aquarius is..."dead" for lucy, lucy absorbs the powers of her friend, committing what can be seen as a "sin")... Perhaps we'll see more of these in the future.
It was enjoyable and all but not my favorite chapter