Just to start off... This was pure PEAK
Holy, this story beca me one of my favorites of the season alongside "A Better World"
Honestly, I don't even know where to begin, but I'll start by saying that I loved seeing Aquaman back. He's one of my favorite DC characters in general, and any story with him will always be a joy. The approach to this one, with the return of Dr. Fate and Solomon Grundy, couldn't have been better.
Solomon Grundy was undoubtedly one of the best aspects of this episode, as it's certainly nice to have been given a backstory and a more complex character than just a strong monster. Throughout the episode, Grundy was extremely helpful and an incredible protagonist, with genuine interest in his soul search, his short-lived friendship with Hawkgirl, and the episode's emotional ending. More than just a Justice League story, it was a Solomon Grundy story.
Speaking of Hawkgirl, her participation in this story was also very notable, both for her effectiveness in combat against the beings from the other dimension and for the Thanagarians' history with Icthultu, as well as the lore given to Atlantis regarding this same entity. Icthultu proved to be an intimidating villain and a huge potential threat if not for the right people to stop him.
By the way, I felt the art in this story was much more detailed than in other episodes, and it really served to highlight the cosmic horror aesthetic of the episode. The entire other dimension was a visual delight and even eerie. I wouldn't be surprised if, had I seen this episode when I was younger, it would have scared me.
Grundy's search for identity was a good theme to explore here, as was the small message about faith in things that occurs with both Diana, Grundy, and as Aquaman himself explains at the end.
It's incredible how emotional the story was at its climax, and with Grundy's ending, I didn't expect to feel this way about a character who hasn't had multiple appearances in the series, and yet I'm still thrilled. What an incredible story! It's currently in my top three of the entire series.