The seismic spear was a long, spear-like weapon that served as Bomonga's Toa tool when he was a Toa Hagah.1, note 1 Bomonga used it to channel and focus his elemental power of earth. Like the lava spear and cyclone spear, it had additional abilities, although it is unknown what they were.2
Bomonga presumably acquired the seismic spear when he became employed as one of Teridax's Toa Hagah3 (all Toa Hagah were equipped with similar spears4). When Bomonga was mutated by Roodaka into a Rahaga, the seismic spear was mutated into a small staff.5 When Roodaka reversed Bomonga's mutation, the seismic spear returned to its original form.CN Bomonga presumably continued to use the seismic spear on Spherus Magna.