The crescent scythe was a double-bladed weapon used by Nektann.1 It was able to access Nektann's elemental power of water2, which he normally could use only in tandem with another Skakdi's elemental power.
Nektann used this weapon during the battle between Mata Nui and Teridax on Bara Magna. During a fight between Nektann and Tahu, Tahu cut the crescent scythe in half by melting through it with his fire sword.3