In HTTYD 3.Grimmel was hired by the Warlords, Chaghatai Khan, Ragnar the Rock, and Griselda the Grievous, because of their problem with dragon poaching to capture Toothless, the only Night Fury he hasn't yet killed. Whilst accepting the offer, Grimmel has his own intentions towards Toothless. He decides to use a female Light Fury as bait for the dragon, freeing it at Berk. He also hides a trap and leaves a poison dart nearby.
Whilst Hiccup is looking through his father's notes, Grimmel approaches him. He walks down the stairs and pours himself a glass of tea before explaining how he killed every Night Fury and shoots a sleeping Toothless underneath a blanket. After Hiccup proclaims that he will never give Toothless up, the Dragon Riders burst out of hiding and into the room, revealing that it was Fishlegs that was shot, not Toothless. Grimmel responds by commanding his Deathgrippers to attack the Dragon Riders and burn the house down with their acid before disappearing with an ultimatum: if Toothless is not surrendered the next day, everything that Hiccup loves will be destroyed. This prompts the Berkians to leave Berk on a quest to find the Hidden World. But unbeknownst to them, they're doing exactly what Grimmel wants them to do. Soon after the Berkians' departure, Grimmel meets the Warlords by the Great Hall, where he reveals he knows exactly where the Berkians are going.
After Hiccup and Astrid return to New Berk from the Hidden World with Toothless, they discover the Light Fury has followed them. It's a moment of happiness that quickly turns around as Grimmel finally takes the opportunity to capture Toothless and recapture the Light Fury. But in an attempt to protect their Alpha, all dragons on the island come to save Toothless. But Grimmel, having foreseen this, points a harpoon at the Light Fury and orders Toothless to call the dragons off. Having succeeded in his mission he leads the dragons to the Warlords, but decides to alter their deal and keep Toothless and the Light Fury for himself. Hiccup and the other Riders surprisingly come flying towards the armada in their flightsuits, starting a big battle. Grimmel's platform quickly gets destroyed and in an attempt to get away he puts one of his collars on the Light Fury, which injects her with the venom of his Deathgrippers, gaining control over the dragon. Hiccup frees Toothless and starts chasing Grimmel down. But Grimmel is prepared and fires one of his darts towards them. Hiccup evades the dart by jumping off Toothless, but this results in the dart hitting Toothless instead, prompting Toothless to fall towards the ocean below. Hiccup kicks Grimmel off the Light Fury and seeing Toothless is falling, removes Grimmel's collar. He tells her to save Toothless and lets go, so he and Grimmel fall to their deaths. The Light Fury saves Toothless and comes back for Hiccup, who removes his prosthetic leg to separate himself from Grimmel. The Light Fury carries Hiccup to safety while Grimmel hits the water, which results in his death.
Role in SAF series.
However, it would not turn out to be so simple, as Grimmel was rescued by His Death Grippers, vowing personal revenge against the boy "That disrupted me", the revenge being strong enough that it earned the attention of a Kingdom Hearts Darkness Slug, a surviving creature from the events of Night of the Night Furies that swam in the Darkness Ooze, and proceeded to attach itself and infect Grimmel, pocessing him and taking full control of him, which in term effected the Death Grippers as well, as the reminant Darkness Slug fuzed forever with the already dark and sinister being, Grimmel internally transforming from an already ruthless man, into what Organization XIII refers to as "Taint Hearts", beings driven by Darkness Slugs to invoke darkness in all living things. Thus, the Darkness Slug had Grimmel return to the humiliated warlords and their armies, turn the trio that were about to scold and turn on him into Heartless, and subugated the surprised armies to his will, as the Darkness Slug took complete hold. Also, to correct a contridiction about claims of him killing off every single Night Fury, Episodes like Drago's Revenge would show that A, he was only a contributing factor when the Night Furies were already suffering deline from a surviveal war with Blackhorrors, and B, reminant flocks actselly both retreated in secluded sactuaries like the Hidden World, giving Grimmel a false sense of superiority in thinking he actselly wiped out an entire spieces said to be extremely smart. Reguardless, he does hold at least some respondsabiliy for their over all absince, ergo, he would be the biggest thorn to the side of peace between Dragons and Humans in the HTTYD World. And now, he is made worse.