Helm of the Conqueror
Wondrous item, rare
While wearing this helm, successful attacks against large creatures deal +4 damage, successful attacks against huge creatures deal +8 damage, and successful attacks against gargantuan creatures deal +16 damage.
Curse. Once you don this cursed helm, you can’t doff it until you are rendered unconscious in battle. It doesn’t really deal any bonus damage, but it appears to you that you are dealing incredible damage to the larger creature before you. When encountering an enemy, you make a DC 20 Wisdom (Insight) check to determine your capacity for defeating it. On a failure, you are sure it is within your capability, with the helmet’s help.
I’ve had “helmet that makes you think you can defeat any enemy you see” on my list for a while now, but haven’t taken the time figure out how to execute on the idea. I think this is an interesting way to do it.