The crystal of Zin-Malor can never be returned to the temple which, as you know, lies in ruin. But I believe that your effort to undo the evil of the past should not go unrewarded.
Seek out Arch Druid Fandral Staghelm in Darnassus, and bring the crystal to him, <name>. It should be in the hands of those that will not abuse its power. From the looks of it, only a handful of the strongest magic users would be able to control such a power.
I have heard of the Crystal of Zin-Malor; the user of the crystal becomes capable of incredible arcane power, but only if truly worthy. I am impressed that you not only recovered the crystal, but you also realize your own limits, and did not attempt to use the crystal yourself.
Thank you for bringing it to me, <name>. We will keep it safe, and may the cursed spirits that once guarded the shattered crystal finally be at peace.