The Atlantean purists are a group of extremists who believe themselves to be the direct descendants of the "real" Atlanteans. They consider any Atlantean with inhuman attributes to be impure and refer to them as "fish-heads". Under the guidance of their leader, Ocean-Master, they seek to purge Atlantis of such aberrations.
The purists convened with Ocean-Master in his lair and brief him on the incident. They say that had Aqualad not intervened, they would have marked everyone as "impure". At his command, they departed.
Poseidonis September 6, 23:45UTC-2
The purists and Ocean-Master infiltrated the Poseidonis palace and besieged Queen Mera. They abducted her, and brought her to their cave while Ocean-Master held off the pursuing Aqualad.
At Ocean-Master's instructions, purists laid an ambush for Aqualad, Superboy and Miss Martian. Though their enemies got reinforcements, Ocean-Master used a spell that weakened the impure. Had Topo not intervened with his ink spout, the purists would have won. Instead, they were defeated, and the stragglers dispersed.
Ronal made his way back to the cave, and witnessed Ocean-Master explain his true plans to Mera: not purity, but civil war to weaken Orin's rule. Ronal attacked him, and soon got help from Aqualad and the others. Eventually, Ocean-Master was forced to retreat.