Quietly brilliant

The pacing is slow and thoughtful, so not everybody's cuppa, and can be surprise for the average anime fan I guess. In many ways it reminds me of Mushishi.

The animation is gorgeous if not flashy. Music is perfect but understated, always taking the back-seat to the visuals.

It tackles the subjects of both impermanence, most obvious example being the death of heroes, and permanence, most obviously through the legacy of heroes. It also brooches, quite brilliantly, on the themes of humility, and true strength.
On a more grounded, and political, level, demons are openly instrumentalizing human empathy to exploit and deceive them. "Demons learn the tongue of humans only to deceive them." "Why do you say this word?" "It's a magic word. When I say it you do not kill me."
The Japanese are naturally based.

Which is funny, in a way. Western culture (what passes for nowadays) has adopted pervasive moral relativism; well, the self-proclaimed mainstream has, and has imposed it on the soft belly of the IQ curve. At the same time, the Japanese, and Asians in general (in the US understanding of the term, not Yookay's) are savagely defending essentialism: what is evil cannot be good, and there is no misunderstanding.
They're literally doing us better than us. Making us look like right muppets in the process. Cheeky tossers.

Frieren herself is a new - at least to me - mix of cute elven girl (sans the ADHD) and 70s movie stone-cold bad-ass. Think Eastwood with longer ears. Being high level is not only about overwhelming power. Although the series features some high-powered fights to rival Dragonball. And Macross too, with its missile saturation attacks. Beautifully animated.
As for Himmel, well, he's a hero of the common folk. He's dead from the start, but damn well present.
By the way, if you know a modicum of German, the names are unimaginative and on the nose. They might be exotic for a Japanese audience, but they rather break the immersion for me.
Anyway, the characters - all of them - are solidly written, with adult and rather subtle arcs, if somewhat predictable. Should be a treat for the teen crowd. By treat I mean a good start for moral analysis and discussion.

Good fiction explores important themes; great fiction leads one to explore them by oneself.

Frieren is a decent shot at greatness.