(This is my prediction on what happened to Toothless and his shudders “family” after the events of Httyd Homecoming. Hiccup’s actions had dire consequences to them, releasing a bunch of apex predators overnight into a new enviroment would inevitably cause chaos and disparity and here is one instance of that.)
Toothless and the light fury began to starve when food supplies ran short. The dragons no longer listened to Toothless’s command and could not defend themselves from the larger dragons, so they took to hiding. Eventually, the light fury turned on Toothless. Not wanting to fight her, he flew up to the same giant crystal where they asserted their divine rights to rule over the hidden world- where, eventually, Toothless ended the light fury’s life. He got a good grasp of her neck and smashed her against the crystal, sinking his teeth deeper and deeper into her neck until her yelps became whimpers, and her whimpers became gags, and her gags became silence. Once she was officially dead Toothless ate the corpse, not wanting waste the oppurtunity of food. Her flesh was soft and unchallenging to rip into. He became so upset with himself he decided to leave the hidden world. As he flew towards the opening, all the way up, he felt the sea breeze and could see the stars of the night sky calling him home. Toothless began to grow excited about the prospect of returning to Hiccup-but, suddenly- his prosthetic gave out. The moon and the stars grew rapidly distant as Toothless fell helplessly down towards the abyss of sharp rocks below.