Thri Kreen Great Weapon Mastery

Living in dry deserts and learning to fight off underground creatures that some could never imagine, you've adapted your body to the abilities of the great weapon fighters of those from the surface world
You've learned to put the weight of a weapon to your advantage, letting its momentum empower your strikes. Using one of your lower, smaller and usually weaker arms to carry the back weight of weapons others of your kind may not use. You gain the following traits:

• Sacrificing one of your usually 4 attacks, you can carry a heavy weapon with 1 of your upper arms and 1 of your lower arms at the same time, reducing down to 2 attacks instead.
• On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.
• Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage.