Morrowseer is the main antagonist in Wings of Fire: The Dark Secret because he captures Starflight and tries to kill him.
The Dragonet Prophecy
He goes to see the dragonets and orders Glory to be killed. Later, he is revealed to be allied with Blister to make sure she's the sister that survives and wins the war. After Blister kills Kestrel, Morrowseer kicks her body off a cliff and tells Blister to kill Webs.
The Lost Heir
Morrowseer meets with Nautilus and Blister after the destruction of the Summer Palace of the Sea, in the epilogue.
The Hidden Kingdom
Morrowseer meets up with Nautilus to discuss the back-up plan of using false dragonets to make the prophecy go their way.
The Dark Secret
Morrowseer captures Starlight and sets him with the false dragonets - Fatespeaker, Flame, Viper, Squid, and Ochre - and tests them to see if they are worthy of the prophecy. First he has the false dragonets try to kill Starflight while telling Starflight to survive, which he does, during which he realizes the real reason why Morrowseer wanted Glory dead - because he was afraid she would find out about his plan to take over the rainforest and warn the RainWings about it.
Then Morrowseer has Starflight and the false dragonets fly to a SkyWing outpost, where he tells them (except Fatespeaker) to convince them to switch their allegiance to Blister. Starflight chooses not to, remembering how evil Blister is, and Morrowseer has his NightWings kill the SkyWings, which he was going to do either way. Squid says he doesn't want to be part of the prophecy anymore, and Morrowseer exiles him to his supposed death (though he is rescued by the Talons of Peace), and decides to capture Tsunami and use her for the prophecy instead, although she defeats the three NightWings that attempt this.
For the third test, Morrowseer tests the dragonets' battle skills by telling them to try to kill each other, saying he'd step in if he thought he must, but making clear he would be willing to lose Starflight or Fatespeaker. He makes Fatespeaker fight both Flame and Viper at once, and Starflight enters the battle and stops Viper from stabbing Fatespeaker in the neck with her tail, causing Fatespeaker to scratch Flame in the face instead. As a result, Flame knocks Viper into the lava. Morrowseer does not want to lose Flame because he is the only SkyWing he has for the prophecy, and decides to capture Sunny to replace Viper.
Eventually, Starflight, Fatespeaker, and Glory rescue the RainWings and convince the NightWings to move peacefully into the rainforest with Glory as their queen, following the death of Queen Battlewinner. Morrowseer is the only NightWing who doesn't accept this plan, and confronts Starflight, Clay, Tsunami, Sunny, and Fatespeaker at the entrance to the tunnel leading to the rainforest. Morrowseer reveals that he and Battlewinner made up the prophecy as part of their elaborate plan to take over the rainforest, choosing Blister so she can be their ally in this. Morrowseer tries to kill the dragonets, but then the volcano erupts. The dragonets escape through the tunnel, but Morrowseer is apparently killed in the explosion.
He is mentioned in the prologue.