Spoiler info

Quest. Talk to Sandra, then talk to Velena.
After that you can choose to search for the place by yourself 
(you will need “Eye of Beholder” spell) or ask Iss for help.
Defeat the miniboss. I recommend you come prepared, maybe with a couple of potions. 
After you learn new spell it will be automatically cast when needed. 
You don’t need to cast it by yourself.
Scene with the boss can be repeated after the quest if you want.