Qual is an enemy in the world of Frieren: Beyond Journey's End (Sōsō no Furīren). He is clever and overpowered but in a good way.
Being overpowered is often considered a bad thing, one that takes away from the enjoyment of anime. That is not the case with Qual.
The third episode of Frieren: Beyond Journey's End has the titular character Frieren defeating Qual, a demon devoted to the Demon King, who Frieren and her friends defeated several decades prior. However, they did not manage to defeat Qual at the time; they could only place a seal upon him. In other words, Qual might have been more powerful than the Demon King himself.
Power is not Qual's only asset; Qual is also extremely intelligent. He created Zoltraak, a killing spell that was impossible to defend from at the time. That is what made him overpowered.
The critical part is that Zoltraak was powerful at the time when Qual was sealed away. While Qual was sealed, the mages of the world lost no time in making sure that newer, better spells and shields took into account the power of Zoltraak. As a result, when the seal was broken so that Qual could finally be killed, Zoltraak was easily held at bay by Frieren's companion, Fern, who is just a human apprentice in terms of magic.
"I love Qual. So OP he became the meta, then was powercrept to oblivion," a fan mentioned on Reddit.
Even then, Qual was not easy to defend from. He quickly figured out how new defense spells were made and immediately understood what their main weakness was. He figured out that such a defensive spell would be costly in terms of mana and decided to exhaust Fern by attacking her quickly and a lot.
Fortunately, he still did not know much about new magic. He was surprised to see Frieren fly, and then, Frieren simply killed him with his own spell, which he also commented on, finding some irony in it as he was fading away.
Qual was obviously overpowered, and in the end, he mages of the world managed to "powercreep" him, that is, introduce a lot of new, better spells. In the end, Zoltraak became nothing but another spell taught to battle mages that existing magical shields can protect from.
Qual is an OP villain that is handled well because the process of making his spells irrelevant appears to be very realistic, but it does not take away from his ingenuity. He is shown to be smart, powerful and a worthy opponent, but he is put at a disadvantage by the seal that held him for many decades, which makes his destruction realistic as well.
A minor point, but Qual seems to have been a loyal demon since, in the episode, he wishes to take revenge on Frieren for killing the Demon King. It is a shame that, as a result, Qual will no longer appear in the series. He was an interesting opponent, and he could have been an interesting companion, but unfortunately, he was too attached to the Demon Lord.
Source: Reddit