Alright, I started to notice a bit more popularity in the original mod ever since I explained what the mod actually included, so I am going to do the same here. I did notice that some people are adding to the wikis, which I am extremely grateful for, and I may even start adding some things of my own into them. The only wiki information I found for this add-on mod though, was for the Treant. Okay time to get into it.

Demon: Looks like a walking version of the demons you find flying around in the underworld. Press Z to to make enemies around you that are on fire to explode. At the cost of lava immunity, press X to make a ring of fire around you or another player and deal fire damage to nearby enemies. You have natural wings with flight time that scales by max HP. You have natural lava immunity that scales with max HP, and is drained by water and replenished by lava. Mana stats apply to ranged and melee damage as well, along with shooting a demon sickle on use, requiring mana. Immune to burning and fire.
+360% Lava Immunity Time | +10% Magic Damage

Formian: Looks more like a beetle than an ant, overall resembles an insect. Press Z to highlight tiles for your ants to mine (OP for chopping wood early-game). Press X to summon a flying ant that can reach a nice distance far from your character, it can be used to interact with things as if you were the ant (such as opening chests, picking up items, talking to NPCs, etc.) but CANNOT use weapons or tools from your inventory, press the down key to stop moving (It follows your cursor). Press C to remove all highlighted tiles for mining. You gain 1 ant per minion slot. Immune to suffocation (Last time I checked this wasn't true).
-25% Endurance | +1 Max Minion | -25% Mining Delay | +25% Tile Placement Speed

Gargoyle: Looks like a gargoyle, has gray stone-like skin and wings. Press Z to enter Glide-Mode and dash towards your cursor, the dash power depends on wing time and critical hits reduce the cooldown. Press X to mimic a statue and lose aggro while buffing defense, leaving this state confuses enemies (reverses their movement). Night Vision.
-10% HP | -100 Aggro | +5% Critical Strike Chance | -15% Mining Delay

Poltergeist: Personally my favorite race this mod has to offer, the poltergeist looks like one of those lost dungeon spirits that drop ectoplasm, and is missing legs. Press Z to enter Spirit-Mode and start flying towards your cursor (beware colliding into blocks, as you will instantly revert and will have wasted your ability until the cooldown ends), flight time depends on wing flight time, and colliding with an entity allows you to 'possess' them, thus being invincible for the duration of being inside while still being able to use items, possessing a player allows for indefinite possession time, does not work on players if pvp is activated. Press X to create a telekinetic ring at your cursor with a custom mana bar that recharges over time, all entities (except bosses) will be sucked towards the cursor, and any items affected will be added to your inventory, which is amazing for mining hellstone with Veinminer. Immune to Poison, Bleeding, and Acid Venom (Again, last time I played this was not true). Immune to fall damage (this part is true and awesome early-game).
-20% HP | +25% MP | +6 Placement Range | +10% movement speed

Prismatic: Looks like a pink light-golem. Press Z to switch the color you emit, red, green, or blue. Press X to create a prism that emits the chosen color in a large radius. Each color gives different buffs, and mixing colors makes more colors that gives different effects. Red increases damage. Green increases life regen. Blue increases mana regen. Yellow increases crit chance. Magenta increases attack speed. Cyan increases move speed. (yeah I am epic, I tested all of this in-game) Doesn't need to breathe (Very good for abyss in Calamity mod).
-35% Endurance | +35% MP

Skeletron Prime: I couldn't put it any better than the commentor named 'milk' who said "Literally the robot skeleton one is just a terminator endoskeleton." (For those who aren't as nerdy as me and milk, it looks like a metal skeleton with red robot eyes.) Press Z to use a selected prime weapon. Press X to select a prime weapon, the choices are Prime Vice, Prime Probe, Prime Saw, and Prime Cannon. The Prime Vice is a grappling-type minion that holds enemies in its claw, and allows for 50% more damage to the grappled entity, press Z to let go of it. The Prime Probe is a laser-type minion that shoots beams at enemies as the retinazor minion from the twins staff does, press and hold Z to charge and release a cursor-following continuous laser stream that DRAINS mana like crazy. The Prime Saw is a tool-type minion that will saw at enemies nearby, press Z to control where it goes, allowing for it to chop trees where your cursor is at, and attack enemies even through walls. The Prime Cannon is another ranged-type minion that shoots slow and heavy cannonballs at enemies, press and hold Z to look around as if you had the sniper scope, and have all enemies around you highlighted. Doesn't need to breathe (yay so useful for wat-). Rusts and takes damage in water (oh...). I would call this race overpowered in pvp, that is, outside of water. I haven't tested this, but I think that the Prime Weapons' damage scales with the current held weapon's damage.
-25% Endurance | +15% Movement Speed

Treant: Looks like a tree, has wood-colored skin and green foliage-resembling hair. Press Z to extend an arm and let out a specialized grappling hook that can hook onto entities and aggro bosses (also works as a regular grappling hook), speed and length scales with max HP. Press X to create a tree instantly on a tile that allows for it, doing so gives the dryad buff to those around during time of tree creation, tree type depends on the tile type (normal grass makes normal tree, jungle grass makes jungle tree, etc.). Sunlight increases HP regeneration. Flowers appear where you walk (basically the flower boots effect at all times). This race is made for a tanky playstyle.
-25% Jump Speed | -20% Movement Speed | +30% HP | +100 Aggro

Wyvarian: Looks like a really short wyvern that decided to become humanoid (Just look at the mod's thumbnail). Press Z to perform a spinning dash that does damage to enemies based on your current held weapon's damage, hitting at least one enemy restores 33% of wing time and halves this ability's cooldown. Press X to create a tornado at your cursor's position, it will lift enemies, destroy enemy projectiles, and restore wing time of players nearby, the size of this tornado scales with wing time. Double tap the jump key and hold it to fly like a wyvern.
+20% mining delay | -10% Damage | +10% Movement Speed | +20% HP

Well there you have it, a somewhat detailed essay on what the races of the mods do. Like MrPlague (I know as I have seen how he has worked with Origins Minecraft, among other things), I have always enjoyed a video-game experience where everyone is unique in both their abilities and drawbacks, and have always loved the idea of being different in my ways of contribution. My goal is to draw more attention to modifications such as these to create more popularity in them. The more popularity, the higher the chances these amazing modders are going to add more and more, creating a ridiculously fun experience for people like me. As much as I wish I knew how to mod like MrPlague and Tianic, I think the best I can do is write the information for them. If all of you enjoyed my essays, tell me and I might start working on the Wikipedias for mods like this. I will not be working for mods that include NSFW content.
Have a good one!