Grimkahn is a major villain in the Shark Wars series and is a giant, wicked Mosasaur. He is the leader of Sixth Shiver, which was created after the death of Drinnok and Fifth Shiver. He is immune to shar-kata attacks on the out side of his body.
Grimkahn first appeared down in the Underwaters with Drinnok and Fifth Shiver. Drinnok disagreed with Grimkahn and said that a shiver can only be comprised of Sharkkind. Grimkahn was seriously annoyed at this. Later on, Grimkahn was freed from the Dark Blue by Hokuu, the main antagonist, and the mosasaurs and frills attacked Fifth Shiver, killing Drinnok. Grimkahn then created Sixth Shiver, with him as the leader and Hokuu as his first. Then, later on, Grimkahn launched his forces towards Gray during the Tuna Run but were buffeted by the oncoming siege of Tuna, and Gray rammed Grimkahn into the current, where he vanished.
Fate: blasted by Gray using his shar-kata powers when he is inside his mouth
Grimkahn eliminated Kendra and Silversun in an easy fight. He was annoyed at Hokuu and roared orders at him. His armada continued to follow Gray until he eventually took out Grinder the hammerhead personally. He was behind Gray's forces but was quickly catching up with them.Gray, Takiza, and even Hokuu found out that Grimkahn could not be killed by shar-kata from the outside. In one battle, Hokuu was tricked into vomiting acid into Grimkahn's face, leading the mosasaur to believe Hokuu was a traitor. Hokuu fled and Gray beat back Grimkahn's forces yet again. When Gray and his friends blasted out of the cave they had been stuck in, the final battlle commenced betweeen Gray and Grimkahn. Gray was swallowed but quickly used a shield and then unleashed a massive fireball from the inside, disintegrating the 80-foot mosasaur.