Grudge is a game mechanic specific to the Ironbreaker. Ironbreakers gain 5 Grudges when they are hit and 10 Grudges when their Oath Friend is hit. Some abilities also build Grudges.
An Ironbreaker can have 0 to 100 Grudges.
Some abilities are more powerful with more Grudges (usually at 25, 50, 75 and 100 Grudges) and some abilities use "Grudge" instead of action points.