Psion

"Psion" is the D&D way of saying "psychic". It is a common opinion that psions in D&D are overpowered. This stems from edition after edition ruining psionics by not separating "mind rape" from "fireball" in the spell department and letting them have both.

In AD&D, psions were having "psionic battles" in the realms of their mind or some shit. Where they would stare at each other and have astral projection arm wrestling and required a bunch of rules nobody wanted to keep track of. It was essentially that episode of South Park where Cartman has a "psychic battle" with the other psychics; it looks that lame to the other PCs.

Third edition or "3.0" had one of the most horrific mechanics ever: the ultimate M.A.D., where you needed a high stat for every subcategory of psionics to have a decent DC. There were no supplements for 3.0 psionics, because there were very few supplements for 3.0 as a whole.

In 3.5e, psionics were generally simple. They were accountant spellcasters who kept track of a mana pool. Rather than spells simply getting stronger as they leveled (though a few powers still worked that way), they had to invest mana ("power points") in order to strengthen their powers. Other players raged about this being h4x because they could essentially screw a BBEG if they really wanted to, but would leave them completely vulnerable later. Most of those who complain about psionics are pro spellcasters, who just want to be the only ones who can do anything at all. With their over 9000 spells per day, their scaling spells just for having moar hit dice, and putting point buy in a dump stat. Psionics actually make you think a little before you can be broken.

But still, 3.5 psionics only had one good class (psion). Psychic Warrior was just a better fighter, (which isn't saying much), wilder a psionic warlock, a soulknife was an even shittier version of the monk WITH LIGHTSABERS11!!ELEVEN11!!! and a few other classes of little importance in the Complete Psionic book (which received very mixed reviews).

In 4e, psionics had their simplest rules ever, even moreso than in 3.x, where whole rule-systems were created for handling psionics in contrast to magic. Psionic classes modify the standard powers diagram (at-wills, encounter, and daily) to instead have a mini-mana system and multiple levels of at-wills. Each psionic class gets so many "psychic power points" per rest period, and can burn these to amp up an at-will to stronger levels. They also split off psychic power types into multiple classes; Ardents were your empathic and psychic healer types, battleminds could tweak their bodies (teleporting, iron-hard skin, super speed) with some mind raping telepathy, and psions had telekinesis, telepathy and conjuration type powers.

It is believed that Dungeons & Dragons' psionics is cursed. That these books are not RPG supplements, but grimoires of a sadistic cult, playing with your emotions. To have any D&D psionics book present in the room will ensure you always roll poorly, and you won't get to sit next to the DM's slut sister who you've been trying to fuck since she was ten.