We first see him when the Ten Commandments are unleashed after being sealed away for 3000 years. He’s the leader of the Ten Commandments, and the youngest son of the Demon King. He’s a flawed character but overall likable especially as time marches on.

There’s a lot of bad blood between him and Meliodas. Zeldris want nothing less than his brother’s obliteration.

When Meliodas tries to become the new Demon King, Zeldris agrees to help. Meliodas promises to free Zeldris’ love, a vampire named Gelda. She had been sealed away due to the Demon King trying to get rid of every vampire.

While he stupidly doesn’t use it during the second season, when he was defending Meliodas, he uses a new technique called Dark Nebula. It makes magical attacks rather useless and brings everything toward him to be annihilated. Pretty darn impressive.

Unfortunately for him, Escanor was able to use his form as The One to defeat him.

He still tries to fight everyone that’s not a member of the demon race until his father returns in his brother’s body.

And when he gets kicked out of Meliodas, Zeldris unwillingly becomes his new vessel.

For which Meliodas and Gelda go inside his mind to free him from control of the Demon King.

When the Holy War is over, Zeldris survives and makes peace with his brother. He goes back to the demon realm with Gelda and takes the throne as the new Demon King. I was happy for him.