Iskos is a spider and is Drina's spirit animal. In the game when you are defeating Drina, he sometimes shoots webs to slow you down. Iskos doesn't use poison in the game, but he uses it in the books.
In the Books
In Rise and Fall, Iskos is said to be a huge spider, and that he was poisonous. As payment for all of Drina's losses to the Keeper of Greenhaven, Iskos, while being controlled by Gerathon, killed Drina. Drina was bitten on her wrist, causing blood to flow loosely. Even without the poison, the book states, the wound would've killed her anyway. It is unknown what happened to Iskos after Drina was killed (he probably died also).
In the first chapter of the first book, Drina summons Iskos in her sleep, causing her to wake up in the middle of the night and scream when she sees him. Shane runs to his sister's room and picks up a lantern to throw at Iskos, but Drina stops him, realizing that Iskos is her spirit animal.
Level 40 Battle
Iskos appears when Drina leads you to a small bridge in the area. When you follow her, you are trapped between Iskos and Drina. Red spots will appear all around you, so it's best to keep moving, as they have a tendency to pinpoint where you were last time. If you get stuck in one of the red spots, your health will go down as Iskos hurts you. There are different places where the red spots will appear so be careful, once Drina gets exhausted she will eventually disappear and you have to dodge the red spots. Later she traps you in webs and you have to run in circles while Iskos tries to trap you with webs. Try not to attack her when she's shoot poison arrows. Your Spirit Animal can but you should try to refrain from doing so.
- In Rise and Fall, Iskos was described to be small enough to rest on Drina's shoulders, but in the game he is shown to be huge (about as long as Drina is tall).
- It's never been stated in the books or the game, but Iskos is most likely a Golden Orb Weaver Spider. They have distinct striped yellow and black pattens, and can grow to be huge.
- Iskos's name could be derived from the Finnish word meaning "to strike, hit"