Complex sentence: The Avatara have become concerned by the emergence of a necromancer calling himself Moagim Reborn, unsure if he is the Moagim of old, or another necromancer using the name, and Leitrim wants Connacht to join in their struggle against this common enemy.
Split into simpler sentences: The Avatara have become aware of a necromancer calling himself Moagim Reborn, and are unsure if he is the Moagim of old, or another necromancer using the name. In either case, Leitrim wants Connacht to join in their struggle against this common enemy.