I always thought the Black Arms aliens were pretty neat, but it wasn't until Shadow Generations where I realized just how fascinating they actually are.
On the surface, the Black Arms were presented as a race of reptilians seemingly taking inspiration from both classic UFO invaders and Xenomorph-like aliens, all led by a mysterious, demonic overlord, Black Doom. With a mixture of sci-fi laser guns and strange biotech, they seek to conquer the world and use humanity as a food source. That's all well and good, but we would eventually learn there's more to these space lizards.
At the end of Shadow's first solo venture, we see the titular black hedgehog defeat Black Doom and destroy the massive alien space vessel, the Black Comet, seemingly putting an end to the Black Arms threat once and for all...
HOWEVER, it is later revealed in Shadow Generations that Black Doom had somehow survived his defeat at Shadow's hands, and seeks to rebuild his forces in order to menace Earth once again.
But how did Black Doom survive? Wasn't he, and the rest of the Black Arms, destroyed by Shadow?
Well, it's revealed that Black Doom was able to transfer his consciousness into a lone Black Arms larva that was kept sealed on the space colony ARK all these years, and from it Black Doom grew himself a new body.
This reveals that not only are the Black Arms a hive mind, but each member of the species is just an extension of Black Doom himself. Basically, Black Doom IS the Black Arms! And so long as even a single specimen of that species is alive somewhere, Black Doom will return.
This even extends to Shadow, being a Black Arms hybrid. In Shadow Generations it is also revealed that Black Doom didn't just want Shadow as an ultimate warrior to use as his right hand. Black Doom's ultimate goal for Shadow was to take over his body and use him as his next vessel, becoming the Ultimate Lifeform himself!
This idea that Black Doom can just grow new bodies from his stock of Black Arms soldiers ad infinitum certainly explains how he's so incredibly long lived! This method of immortality is just so fascinating to me.
What's also interesting is how this hive mind differs from other depictions I've seen. Usually, a hive mind is depicted as each member having no sense of self or individuality. All are equal in their lack of identity. But with the Black Arms there is almost paradoxically a true identity among them, a single entity that goes by a name. An entire species that exists only as Black Doom's will made flesh. Each member, each body, though they all have faces, are all ultimately nothing more than the limbs, or "arms", of Black Doom.