A variation of the Warlock (the ongoing rumored class similar to the Death Knight in WoW) which is thus aimed to be more unique.
Even during the glorious days of the Zakarum some of its members started to believe that mortals were not vessels adequate enough for the Light due to their natural preference towards it. This quality, which was before logically considered to have the opposite effect, was discovered to somehow limit the amount of Light that could be infused with an individual. Thus, a secret project was started with the aim of creating a mortal with a neutral soul and mind. The result were twins whom heart knew nor Evil nor Good, but that would grasp whichever it interacted with first. And so, the children started preparing for their infusion with the Light once old enough.
When Travincal finally fell and the Zakarum dissolved, the children were ready for infusion. However, most of the few remaining members that knew of the project had by then forsaken the Light; not as a purpose, but as a tool. Thus, when the moment came, they chose one of the two brothers and infused him with Chaos. The result was horrible; the boy became an uncontrollable demon.
Once the failed experiment was put to rest, the project was forgotten and the last remaining twin left to fend for himself. He was left deep within a cave, where neither Chaos nor Light would reach him. However, the young one's neutral soul was like a vaccum yearning to be filled and soon his desire guided him to the outside world. Now a young adult, the child must decide his path: Become the greatest Light wielder Sanctuary has ever seen, become a Chaotic demon or master a combination of both. This only time will decide. The only thing he knows is that he is a being. He is a Spawn.
So I hoped you liked the lore, now off to the general nature of the class. Mainly Damage. As you may have predicted, the Spawn is similar to the Paladin BUT it can also be similar to an Assasin (high melee DPS but not so much defence, thats what I am trying to say) or an innovative combination of both, depending on the build chosen. So without further delay,
-Classical use of auras but offensive oriented ones (E.g. Might) of course different names and fewer variety but more powerful
-Increasing surviveability as an alternative tank BUT in a new and interesting way which would involve a system described in a bit of lore "The Light is now so strong within him that it heals him constantly" so not a lot of armor or HP but ways of increasing the amount healed.
-Much more damage oriented, offensive skills that are based around slashing and the like (more info on this later)
-A new system for damage as well which I well know to be popular, where with each attack + dmg + attack speed (but you choose one of the two along the tree to master or both to kinda combine. this allows a lot of possibilities)
Hmm hold on lets explain that. So:
Imagine two passive "skills" in the tree at the same tier, with the same amount of points to spend. One increases the attack speed with each attack while the other increases the amount of dmg with each hit. It would be designed so that you either max one or the other, but you can combine both without maxing. So if you chose both you wouldn't get as much damage or attack speed, but rather a combination. Depends on taste.
-Focus on high dodging chance (like HIGH) over defence and health. Not enough to be an alternate tank though.
-As the name implies, concentration on both but aimed for either DMG or tanking capabilities (not really though considered a tank).
-So the regenerating Light thingy I mentioned would be a focus in this build but a variation. Imagine an empty glass bottle that either gets filled with Sprite or Coke but otherwise it is empty. So, it would rely on probably orbs of concentrated Light and Chaos inside his body which together cancel out. So one orb heals (Light) and the other one damages (Chaos). There would be three skills for him to use in this neutral state: Infuse Light, Infuse Chaos and Gray. So when infuse Light is used the Chaos orb leaves his body and then offensive skills in the form of Chaos-ranged spells can be used while the Light orb heals you. Counter to this, infuse Chaos allows you to use the Light orb outside your body to heal to a certain degree but the Chaos orb would damage YOU overtime. Then Gray fuses both for a rather short amount of time and allows you to regenerate like in the Light tree and attack like in the Chaos tree.
Ok so he would use Claws like an Assasin. Picture the skills with those. Also, if they would actually use this for the game, I would love it if the characters appeareance changed the more it went down the tree. So at the start it would be like this normal guy (or girl i guess) but with white hair and colourless iris (also pale skin). The more points spent on the Chaos tree the more the character will look like a half-demon or something but not much, still looking human, but with shining red eyes and black hair, etc. Light would be blond with yellowish shining eyes. Both representing that the guy/girl is overflowing with the Light/Chaos. But if they went Twilight then they would stay the same, only the orbs would vary.
Complicated, I know. It wasn't in my head when I started writing this lol.
Note that the skills in each tree are not named, but I kinda described what they would aim for in conjuction. You guys can help me with names and specific effects if you liked the class.
Finally, I know that at first this seems to disobey the normal parameter of mortals not being sided with Hell or Heaven but consider this; neither is the Spawn. He merely uses Chaos and Light like tools, but his purpose, like any other mortal in sanctuary, is to survive.