Spoiler level: Very low, most of it is in the synopsis anyways. First episode at most.
Fantasy, action, and imagination come together to create a sense of depth and realism that is very uncommon to see in anime and even other platforms. The story takes place shortly after several people teleported into a different dimension, recruited into a militia force, and split up into two groups: the front liners and the newbies. The entire situation is very similar to that of <every MMO ever>, including the way the initial mysterious event that started the show quickly becomes irrelevant. What's unique about Grimgar of Fantasy and Ash is that the story is focused on the latter and not the former, and it is focused around adjusting to life in the world rather than why or how they got there. In short, it is an uncommon take on fantasy and video game–inspired worlds, and the concept is not left without a follow through.
It took a skilled hand and a brilliant mind to create the fantasy world of Grimgar. At first glance, the art looked messy and strange—very different from most other anime. The world is drawn in the same style as one would think a light novel or manga. There's something unique about the lines and colors that make up the characters and environment; it really drives home the contrasting themes of fantasy and reality. The visuals are pleasing to the eye, and they were created with a keen sense for detail. Some notable details are the parallax effect in certain scenes, the night sky effects(by the gods, look at the sky when you get the chance!), and finally the particle and animation quality. Watch the entire opening at least once to get an idea of how good the animation is; watch it again every now and then, and notice how it changes with how many people are currently in the party. On top of that, the characters are unique and well drawn, and it won't take long before identifying who's who is instinctive.
When music and sound are done well, the atmosphere and theme for the entire show can be set just like that—a snap of the fingers. The only thing I need to say is that this is one amazing sounding anime. The soundtrack is great and theme appropriate, with a good variety in music for different scene types. For the opening, there's hope; for the ending, there's serenity. Battles are made many times more enjoyable with the epic music. I could go on, but even if you don't plan to watch the show, at least give the soundtrack a try.
A (literally) colorful cast, each with a personality and role in the party, all told from the perspective of the thief, a withdrawn type who is thrust into the leader position without warning. None of the characters seem static or uninteresting. There is at least some characteristics that change over time, even if they are slight. The aforementioned thief, Haruhiro, tells the story from his point of view; as a result, there is no reading the minds of others, nor can we see things that they cannot. The effect this has on the show is to put all importance on their party, rather than the rest of the seemingly vast world. While the anime could have become something about Grimgar on a global scale, or the power players whose appearances are close to zero, the studio chose to keep it down to earth, and the closest to the ground happen to be this party of six.
In two words (not counting the conjunction): captivating and suspenseful. Personally, I found Grimgar to be extremely enjoyable. I myself am knowledgable in the field of video games, and I can say that watching this anime was a welcomed step in the other direction of competitive or professional gameplay. Watching these characters grow from the first episode to the twelfth helped strengthen an emotional bond, and although they started out giving me facepalms (come on, use some strategy! it's a goblin, for cryin' out loud!) I eventually came to like their synergy both in battle and out. Also, I loved listening to the music and gawking at the visuals.
This show is a worthwhile watch for both gamers and non-gamers. Overall, this show does not disappoint in any field, and I am legitimately glad that it gave me all the reasons to argue that Hai to Gensou no Grimgar is a masterpiece. Of course, enjoyment and opinion vary from person to person, so by no means am I trying to force my own thoughts upon anyone else. However, I truthfully believe that this was one of the best anime of the season, if not one of the best I've ever watched, and I want to share my reasoning in the hopes that I could convince some other people of the beauty packed in these twelve episodes of twenty-or-so minutes.
