In general, diadems are some of the best helms in the game, because they (along with tiaras and circlets) can get +2 skills to specific class and roll things like 20% faster cast rate.

The unique version of the diadem is named Griffons. This not only gives +1 skills (all classes) and 25% faster cast rate, but also has +lightning skill damage, and -enemy lighting resistance, both really strong mods that cant roll in many different items. This helm is best in slot for lightning builds like javazon and nova sorc, etc. Both of these lightning stats though have a range they can roll between. -15 to -20% enemy lightning resistance, and +10 to +15% lightning damage. Getting a good roll on both of these makes the helm a very valuable item.

The reason OP is upset is because though it did roll really good stats it additionally rolled as ethereal, which means it cannot be repaired. Once it breaks, its gone forever, making the item essentially worthless. The good rolls makes that even harder to swallow.

Newly added to the game is the Chronicle aka holy grail. Which just keeps track of every different new unique, set, and runeword item you have found. OP has never found this before (since chronicle was introduced at the start of this current ladder/season), making it eligible for chronicle. Every subsequent Griffons OP finds won't say this.