So, you have chosen to capture the soul of an infernal, good.$B$BTake 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.$B$BI would suggest going to Desolace and trying your hand at any of the infernals there. Those should do nicely.$B$BAfter you're successful, take both the captured spirit and your emerald to Menara.