Complex sentence: Unlike the previous games, the Rage meter is filled by attacking enemies and once full, the ability is automatically activated and remains activated until Kratos is attacked (or stops attacking for an extended period of time), then requiring the meter to be refilled.
Split into simpler sentences: Unlike the previous games, the Rage meter is filled by attacking enemies and once full, the ability is automatically activated. It remains activated until Kratos is attacked or stops attacking for an extended period of time, then the meter must be be refilled.