If this Pokemon would take damage from Stealth Rock, it is instead healed by 25% and that entry hazard (but no other) is removed from this Pokemon's side of the field.
#2: Mad Dash
The Pokemon this ability is exclusive to has a wide movepool, base 100 in Attack and Special Attack, and base 95 Speed. It also gets Light Metal, but has a weight such that it does nothing. (Essentially, it's this or nothing.)
Each move used by this Pokemon has one added to its priority, but this Pokemon's Attack and Special Attack stats are halved.
So, my two questions regarding these abilities are, "What tier would Regirock be in with Rebuild?" and "Would the Pokemon described in the Mad Dash description be better off with Mad Dash or Light Metal?".
Generation (pi)(i). When you see this, raise e to the generation and add 1. Social experiment.