Shard of an Infernal
Take the Infus Emerald and use it on one of the Infernals found in Desolace. After successful, bring the Infus Emerald and the Imprisoned Infernal Spirit back to Menara Voidrender in the Barrens.
So, you have chosen to capture the soul of an infernal, good.
Take this emerald--with it, you will be able to capture the creature's soul. It works very similar to spells you already know: when the creature is close to dying, use the gem and it should do the rest of the work for you.
I would suggest going to Desolace and trying your hand at any of the infernals there. Those should do nicely.
After you're successful, take both the captured spirit and your emerald to Menara.
Ah, the spirit of an infernal. Well done, <name>.
When I have both the spirit and the cleansed orb, I will merge the two for you, but not until then.
Upon completion of this quest you will gain:
The entirety of this quest happens at The Barrens