Shield Spikes

When added to your shield, these spikes turn it into a martial piercing weapon that increases the damage dealt by a shield bash as if the shield were designed for a creature one size category larger than you (from 1d4 to 1d6, for instance). You can’t put spikes on a buckler or a tower shield. Otherwise, attacking with a spiked shield is like making a shield bash attack (see above). An enhancement bonus on a spiked shield does not improve the effectiveness of a shield bash made with it, but a spiked shield can be made into a magic weapon in its own right.

Damage (Light Shield): 1d3(S), 1d4(M) /x2 Critical
Damage (Heavy Shield): 1d4(S), 1d6(M) /x2 Critical

Cost: +10gp
Weight: +5lb

Source: Player's Handbook