Your prowess with a specific martial weapon is beyond compare.
Benefit(s): Select one martial weapon you are proficient with. When making additional attacks as a result of a high base attack bonus, each attack after the first has its attack bonus reduced by 4, rather than by 5. This does not change the number of additional attacks you gain, only the attack bonus used by each additional attack.
For example, Jacinth is a 14th level fighter proficient with the longsword who has taken Mythic Martial Weapon Proficiency with that weapon. Normally he would receive three attacks as a result of his base attack bonus, at +14/+9/+4. However as a result of this feat, his three attacks are instead at +14/+10/+6.
Mythic Options: The Missing Core Feats. © 2013, Owen K.C. Stephens; Author: Owen K.C. Stephens