Kheldians are energy beings from outside our galaxy. They have the ability to join with human hosts in a (usually) symbiotic relationship. When this happens, the two separate personalities merge into one new one with its own identity and name. There are three types you'll typically encounter in the City of Heroes universe: Peacebringers, Nictus and Warshades.
Peacebringers are regular Kheldians, beings composed of light. The Peacebringers themselves are dedicated to hunting down the Nictus and destroying them for what they did to the Kheldian race (and still plan on doing.)
Nictus are Kheldians who, when faced with the short native lifespan of the Kheldian people, turned to science and changed themselves into beings of darkness. They feed off of life energy, and were at one point able to siphon the life force of the Kheldians from great distances.
"I have told you that the Nictus once fed upon other Kheldians. What I did not tell you was how. The Nictus scientists developed a powerful energy transfer device that could rip away a Kheldian's life force, even at a distance of light years. This terrible weapon is the reason we Peacebringers have resolved to destroy all Nictus, wherever they may be." - Moonfire
Warshades are "redeemed" Nictus. These are Nictus that have had a change of heart, merging willingly with another instead of trying to force their way in. They fight alongside the Peacebringers against the Nictus, and have a sometimes uneasy truce with them.
Most beings are, for lack of a better description, like apartments to Kheldians. They can move in, if the host is willing (or very weak,) and they can separate at will. They cannot be forced into someone (again, unless they're very weak, or "boosted" in as Arakhn tried to do.) And with enough willpower, the host can force them out. No 30 day notice needed.
The joining has advantages for both the host and the Kheldian.
The host gets power - typically seen in-game as attacks, shields and the like. In addition, it seems the host's natural lifespan is increased as well.
The Kheldian, as mentioned, stretches its lifespan when joined. In addition, in the future it can make an energy "copy" of any being it's been joined with in the past - such as the Nova and Dwarf forms of tri-form Kheldians. (So, yes, in the future a Kheldian from Paragon could merge with something else and change to Human form. Or alien, or catgirl, or whatever else it merged with.)
The two personalities merge together into a new single entity. In the case of Peacebringers and Warshades the personalities influence the whole. Nictus, on the other hand, do their best to dominate the host and take complete control. It is possible for the Kheldian to depart the host and the host can actually force a Kheldian out with enough focused willpower.
Being energy, Kheldians have no natural "shape." At best they appear as wisps of energy. OK, occasionally bad tempered, deadly wisps of energy, but wisps nonetheless. You can see this if you run across a Shadow Cyst - it'll spout "puffs" called Unbound Nictus that will do their best to kill you. They're pretty good at it when they get going.
We don't know how Khelds reproduce. Nictus, however, have a means apparently available to them (hinted at in the Galaxy soldier section.) In order to make a Galaxy soldier, N-fragments ("pieces" of a Nictus) are implanted. We don't know how many pieces it takes, or how many you can get from a dying Nictus. 40% of those troops (at this point) opt to become full Nictus. If you can get more than one set of shards from a Nictus - and given there's a bit about creating them from healthy Nictus, as well - you can have multiple Nictus created from one. How much of the original Nictus personality remains is unknown.
Peacebringers seem surprised by this, so they most likely have some other means available to them.
We don't know of "native" Kheld gender. The hosts obviously have gender, but native Khelds - unknown.
Alignment - Any however the category of alignment will determine some the power set available to the Kheldian. Good alignments are Peacebringers, Selfish alignments are Warshades, and Evil Alignments are Nictus.
Nova (Squid) Form - IQ: 3d6, MA: 3d6, ME: 2d6+2, PS: 2d6, PP: 4d6, PE: 2d6, PB: 2d6, SPD: 3d6
Dwarf Form - IQ: 3d6, MA: 3d6, ME: 2d6, PS: 4d6+6, PP: 3d6, PE: 4d6, PB: 2d6+2, SPD: 2d6
Human Form - IQ: 3d6, MA: 3d6, ME: 3d6, PS: 3d6, PP: 3d6, PE: 3d6, PB: 3d6, SPD: 3d6
Nova (Squid) Form - As the name implies this looks like a normal sized squid. The only drawback is their obvious eregy being appearance.
Dwarf Form - Much like the dwarfs of fantasy, these beings look stocky and barrel chested except in a very oversized body (think Incredible Hulk size). However like the squid form, they are obviously energy in appearance.
Human Form - The ubiquitousness of the human form is everywhere throughout the galaxy. The human form of Kheldians is unique among their 3 forms though in that they are not obvious energy being with only glowing eyes giving that away.
ALL - Energy Resistance: Energy attacks do only half damage. Regeneration: "Heals" 4d6 (Nova), 6d6 (Human), or 1d6x10 (Dwarf) SDC/HP per melee round but must use an attack to do so.
Nova (Squid) Form - Super Energy Expulsion, Extraordinary PS, Sonic Flight
Dwarf Form - Hardened Skin, Impact Resistance, Extraordinary PE, Weapon Energy Extensions, Supernatural PS
Human Form - Energy Expulsion - Energy, Personal Forcefield, Energy Shield, Flight, Superhuman PS
Peacebringer - Karmic Power, Force Aura
Nictus - Distruptive Touch, Force Aura
Warshade - Teleport, Cloaking
Nova (Squid) Form - PE plus 1d4 plus power bonuses
Dwarf Form - PE plus 2d6 plus power bonuses
Human Form - PE plus 1d6 plus power bonuses
Nova (Squid) Form - 1d4x10 plus power bonuses
Dwarf Form - 3d6x10 plus power bonuses
Human Form - 1d6x10 plus power bonuses
Armour Rating (Natural)
Nova (Squid) Form - 12
Dwarf Form - 16
Human Form - 14
The native lifespan of an unjoined Kheldian is 10 years. Some have said the Nictus lifespan was reduced to 5 years, but this is unverified.
Kheldians can, however, merge with a host being. They aren't (typically) parasitic, though the Nictus often seem to try to take over the host's mind. With a continual chain of hosts, Kheldians are functionally immortal. (Shadowstar, the Warshade leader, has been around since ancient Egypt - 3000 to 5000 years.)
Nictus, after traveling across vast distances via Shadow Cysts, tend to be weak and use the cysts as "life support" if no host is ready. If the cyst is destroyed, the Nictus die off quickly, regardless of age.
Size - Equal to the host form, except the Dwarf which is in name only. Dwarfs are 9-12 feet tall.
Education Level - Any and/or choose a class from the following: Hunter/Vigilante, The Secret Operative, The Super Sleuth, Analytical Genius, The Mega-Hero
Note - Mega Hero can be chosen as a stand-alone or add-on.
Paragon Archive Wiki for City of Heroes and City of Villains
Heroes Unlimited RPG and Sourcebooks
Rifts Dimension Book: Skraypers