The Spirit Army
To progress the game to Act II, you must leave Mulsantir and arrive at Mulsantir Gate. Be sure to bring along a full party, cause you’ll need all the help you can get.
You’ll soon find Okku and army of Dryads and Telthor animal spirits waiting for you. Gann will fill you in. Okku is invincible while drawing strength from the spirit army. You must first cut the army down to size considerably before Okku is weakened enough to strike directly at him. Gann will let you know, but if he’s killed then it’s a matter of using your ‘best judgment.’
Go all out with area effect spells like Chain Lightning, Meteor Swarm and Wail of the Banshee (yes, this will work on Telthor spirits). You need to reduce the numbers as quickly as you can because Okku can dish out terrific melee damage and you want to make this fight as short as possible. Once Gann gives the signal, or you judge the time is right, go all out for Okku.
He’ll surrender once he’s taken enough damage. In Okku’s presence, your mysterious hunger reawakens and confronts you with 2 choices.
One choice is to control your hunger and spare Okku and recruit him into your party as a tank. This implicitly commits you to taking the good path in Mask of the Betrayer since Okku will have a strong, often violent, aversion to many of the evil choices that are available in the game, especially those that concern how you deal with your new status as a spirit eater. (more on this below)
The other choice is to devour him as a Telthor spirit. This yields Okku’s Essence, which can be used to enchant a belt to provide a +10 bonus to Strength, and Okku’s Husk. This implicitly commits your party to taking the evil path since it marks a willingness to indulge your new spirit hunger and also allows you to later on recruit a thoroughly evil companion.
Either way, you’re now the newest Spirit Eater of Faerun. This status is not only integral to the plot, but understanding how to deal with it is vital to your very survival. For more on it, see this link.