Complex sentence: In the film, Radagast is the first wizard to visit Dol Guldur after he realises an evil power has infected the wood he lives in, and discover that an evil power, a "Necromancer," has established itself there.
Split into simpler sentences: In the film, Radagast is the first wizard to visit Dol Guldur after he realises an evil power has infected the wood he lives in. He discovers that a sinister entity, a "Necromancer," has established itself there.