A little experiment in unique mechanics, I was looking at how to give bonuses to a weapon that would help with high-level opponents, without being overly powerful against low-level enemies, and thought of modifying the enemy AC rather than adding a flat bonus to attack rolls.

Unless you're fighting the Tarrasque, you're never going to get a massive boost from the Undeniable ability, but it will be useful against some of the larger dragons, opponents with heavy armour and shields, magic items of their own, or making use of mage armour and other spells.

Against lower AC enemies, the expanded critical range will still come in handy and making it out of adamantine just seemed cool and added some extra versatility for taking out extra tough enemies (such as golems and helmed horrors), which is the primary theme of the item. While it's status as a magic item would technically bypass those resistances anyway, it seemed like a good bit of flair to add and may come in handy in a pinch when you're hit by an anti-magic field.