Armor Spikes

You can have spikes added to your armor, which allow you to deal extra piercing damage on a successful grapple attack. The spikes count as a martial weapon. If you are not proficient with them, you take a –4 penalty on grapple checks when you try to use them. You can also make a regular melee attack (or off-hand attack) with the spikes, and they count as a light weapon in this case. (You can’t also make an attack with armor spikes if you have already made an attack with another off-hand weapon, and vice versa.)
An enhancement bonus to a suit of armor does not improve the spikes’ effectiveness, but the spikes can be made into magic weapons in their own right.
Damage: 1d4(S), 1d6(M)

Cost: +50gp
Weight: 10lbs

Source: Player's Handbook