The Death of Superman was a quality DC animation; but while it was a fun viewing, it didn't provide anything particularly new, especially with the wide audience of the live-action Batman v Superman: Dawn of Justice which covered the same content. Reign of the Supermen is effectively a continuation of the same story line, forming a collective 168-minute animated extravaganza. As such the duo have received a limited theatrical release.
Reign of the Supermen starts 6 months after the death of Superman, where 4 new heroes have emerged all claiming to be Superman reborn, and Lois Lane is trying to figure out whose claims are true.
Based on the graphic novel of the same name, don't expect to find a faithful adaptation of the source material here. Apart from names and likenesses in these new characters, the majority of their backstories, and roles in the film are departures from the source. But whereas the graphic novels has generally treated each character in their own isolated arcs, Reign of the Supermen has all four characters story lines intermingling in quite a cohesive manner, connecting back to The Death of Superman and does the work to build up for the next animated release.
With the absence of the Superman that we all know and love, the film primarily focuses of Lois Lane and Lex Luthor, and their characters are looked after really well, highlighting how under-utilised or poorly treated they were in the live-action DCEU. Lois Lane is doing what she does best, and the film thrives with so much of the plot being driven by the human characters. Even Lex Luthor is back with his inexorably charming personality, superbly done by voice actor Rainn Wilson (a.k.a. Dwight from The Office).
The film is incredibly well balanced, introducing four new characters, developing backstory and motivations, all in the space of 87 minutes. While not the strongest of plots, the film still flies with some great pacing and action scenes that ensures there isn't a dull moment. DC has always had a pretty consistent art style and quality; Reign of the Supermen is no different, though admittedly the quality does drop off a little bit when multiple main characters are in the shot, but in general the close-up shots have great clarity and tone.
The action scenes and choreography are a step up on The Death of Superman, but if you get a chance to see them both on the big screen, I implore you to. Reign of the Supermen has the payoff that was missing from its predecessor, and to see it all come together as it does, is a satisfying sight.