Hi there,
I've been a DM for many years, but right now I'm having a bit of a creative block that prevents me from moving the campaign forward.
To sum up the plot: I'm running a Conan the Barbarian campaign with DnD rules, updating from 3.0 etc to 5th ed as we go. I know that's not optimal, but the players are loving it. Anyway, right now the plot is revolving around serpent people having discovered and fled with two stone obelisks from civilized Koth. These obelisks are actually horns belonging to a sleeping god (Ur-Shustu), and the evil guys have caravaned these through the desert and reattached them to the now awakened god.
The thing about this Ur-Shustu is that he's a both a god and a gate. Millennia ago, when the Acheronians were losing, they fled into the desert. There a final battle ensued, and the story passed on through the generations is that the dark god went mad and consumed the last of his people, before the hyborian avatar of Bori defeated the enemy god, broke off his horns and hid them. Actually he did break the horns, but Ur-Shustu didn't consume his people, he sucked them through a portal into a limbo-like plane, I guess it's a bit like Shadowfell, at least a plane where time passes slowly.
My inspiration for this came from this Age of Conan artwork .
My players have fought the serpent people many times, and the most recent confrontation was when they braved the sandstorms of the desert and came head to head with the cultists and the awakened god. They lost in an epic fight, one of the players being sucked through the portal between the god's horns into the dead realm, while spitting defiantly in the god's face, Conan style 
The others got away, and fled back to civilization while mourning their dead friend.
Of course, I'm not going to let it end there. Their dead companion (whose player has accepted the death and is loving his new char as well) will come back from this realm of the dead and become a powerful general (masked), leading an army of returned acheronians who come through the now active portal.
TL;DR
So, my question to you is: How do I make the acheronians from the shadow realm cool, dreadful and everything that makes an evil race something to hate.
Are they dead? If so, are they ghosts? Do they even die in the limbo-plane? Are they incorporeal? Can they practically possess people if the rules say that you should save every day? Etc.
If they return to the world of the living, will they try to stake a claim in civilized lands? If they take over the living bodies of others, are they going to start growing crops, raising sheep (and all those mundane tasks you need to keep a civilization alive), or do they just tell somebody weak to feed them?
My first thought is to have them turned into ghosts/wraiths who can possess people and maybe some of them can be more powerful like liches or vampires. But then I thought of this forum, maybe some of you have an idea for these acheronians? And what would be a really cruel way of getting even on the hyborian nations that defeated them all those years ago.
Ok, hope to hear from you. Any input is appreciated.
EDM75