Complex sentence: He carried a black axe and fought in the Dagor-nuin-Giliath, where he mortally wounded Feanor as well as the Nirnaeth Arnoediad where he slew Fingon.
Split into simpler sentences: He carried a black axe and fought in the Dagor-nuin-Giliath, where he mortally wounded Fëanor. He also fought in the Nirnaeth Arnoediad, where he slew Fingon.