Black Clover has largely kept the nature of the first Wizard King under wraps as all fans know about the character so far is that he defeated a great evil long ago and became the first Wizard King.
But in the latest episode of the series, there's a flashback that important ties the Wizard King to the Eye of the Midnight Sun, and it explains a little of why they're so angry at the Clover Kingdom.
Like when Vetto took a beating during the Seabed Temple arc, Fana's losing battle to Asta causes her to experience a "flashback" to a time that she's completely unaware of. In this flashback, Licht (the current leader of the Eye of the Midnight Sun who Asta helped defeat previously) is revealed to be an elf who had befriended the first Wizard King.
The Wizard King wanted to create a peaceful world for both humans and elves, and Licht even married a human woman and was about to have a child with her. But like in Vetto's flashback, the good times do not last as the Elves are killed by a large light spell. They believe the Wizard King is the one who betrayed them, as he's the only one who knows where they live, and Licht watches in horror as his people are killed mercilessly.
Although there's no concrete connection between the Wizard King and this attack, this attack and the human's perceived betrayal are a major reason Licht hates humanity so much. It's why the Eye of the Midnight Sun seem to attack and lose all sense of themselves within the hatred, and shows that Licht's problem with the Clover Kingdom goes back a long way.
This is also one of the few times fans have gotten a good look at the Wizard King who adorns the intro sequence of every episode, but hopefully it's not too long before it's revealed what really transpired that dreadful day.
Yuki Tabata first created Black Clover for Shueisha's Weekly Shonen Jump magazine in 2015. The popular series follows two young boys named Asta and Yuno who are growing up in a world where magic is everything. Asta, unfortunately, seems to have been born without magical powers while Yuno is a young prodigy whose magical powers rival that of the strongest magic users in the Clover Kingdom, the Magic Knights. The two young boys now strive to become the strongest wizard in the Clover Kingdom, The Wizard King.
You can currently find the Japanese language version with English subtitles on Crunchyroll, and the English language dub can be seen on FunimationNow and Saturday evenings on Adult Swim's Toonami block.