Strike of the Enduring

"You strike an opponent, backed with the strength of the earth."
 * I :	Your attack deals 4 additional damage.
 * II :	Your attack deals 8 additional damage.
 * III :	Your attack deals 12 additional damage, and 2d6 acid damage on critical hits.
 * IV : 	Your attack deals 16 additional damage, and 2d10 acid damage on critical hits.