What is the difference between 诡异 and 奇诡?
Both 诡异 and 奇诡 are adjectives. The "strangeness" translation is incorrect.

诡异 is a commonly used word meaning "strange". It can be used both colloquially and in writing. The idiom "好诡异" you mentioned means "How strange it is!".

奇诡 is a formal word used exclusively in literatures. It is a stronger form of 诡异 and is usually reserved for things that are extremely bizarre or supernatural. Use this word with caution.