One of 6 legendary enchantment tomes. 

enchanting uses 5 of the caster‘s base HP. This loss is permanent. Enchanting a weapon takes 1 short rest while attuned. 

The tome cannot be used by barbarians or fighter

Five times per encounter, the attacker rolls 2d12 instead of 1d20 to pierce armor. Regular modifiers still apply. The enchanted weapon otherwise deals regular damage. 

One enchantment lasts for 10 encounters. After the enchantment wears of, it is a regular weapon again.

This means attacking with a weapon enchanted with this tome results in a 20%chance of rolling, for instance, a armor piercing roll of over 20 instead of a 5% chance.