Ad Astra psionics

Although all characters have a Psionics skill rank (except robots) only characters with the Psionic Abilities class ability can actually use psionic powers. Using any psionic power costs 2 hit points (because it's tiring and wears you out to rely on it too much) and requires a skill check of 1d20 + Psionics + the applicable stat bonus. Sometimes the DC that a character is trying to hit with his psionic check is an opposed psionic check—even for characters who cannot use psionic powers themselves, they still have a psionic skill rank which they can use in opposed psionic checks. Other times, it's a static DC set by the GM.

For simplicity sake, there are only a few listed psionic powers in this basic rules-set There may be other psionic powers available.  There are no rules for them here, if so, but if desired, they can be home-brewed into the game with GM approval and involvement.

When using psionics in combat, you must make your psionics skill check first. If you fail the check, you still lose the hit points—it always costs 2 hp to use psionics, but the opponent would incur no ill effect since the check failed. Some powers can be used against multiple targets. That's OK, but there is a -2 penalty to the psionics skill check for each additional target beyond the first, and it costs 1 extra hit point per each additional target beyond the first. So, for example, attempting to use telekinesis against five combat bots at once would incur a -8 penalty on the check (which must be made separately for each target) and would cost 6 HP regardless of the outcome.

• Telekinesis: Psionics + MND check vs. Physical + STR or DEX (usually whichever is better) of the target, if used as a telekinetic push against an opponent. Telekinetic push inflicts 1d4 damage per attacker character level, and knocks the opponent to the ground. The opponent suffers a -4 to AC until they can use a Move action to stand back up. This can also be used to pick up and hurl objects, or to have objects (such as a dropped weapon) come to you when "called." Hurled objects typically do 1d4 or 1d6 damage (depending on the size) to whomever they hit. It also typically takes both a telekinesis check and a Ranged Attack check to both successfully pick up an object and hit your opponent with it. It's a difficult thing to do, which is why when advanced knights do this, it's pretty impressive. You don't see a lot of Knights trying something that complicated too often.
• Biometric surge: Psionics + MND vs. static DC set by the GM. Useful for the amazing feats of speed and leaping common to Knights. Failure consequences depend on the degree of failure, i.e., if you just miss your check on a Jump, you don't reach your goal and may have to jump again. If you blow it by a larger margin, you may hurt yourself falling in the attempt (see rules for falling in the Combat section.) This one is very situational in terms of what the penalties for failure might be, although a good GM should use swashbuckling fictional source material as a guide and not be overly punitive. This is supposed to be an application of psionics that allows Knights to be superheroic and swashbucklery, and that requires a fairly generous interpretation both of DCs and the consequences of failure or else you'll discourage your Knights from acting like Knights.
• Prescience: Psionics + MND check. For every 10 points on your check, the character gains a +1 to attack, damage and AC for the duration of the combat. For example, if a 7th level Knight with a +2 MND bonus rolls a 10 on a d20 (for a total roll of 19) he would gain +1 to attack, AC and damage, but take 2 points of damage, as normal. If he had rolled an 11, for a total of 20, he would gain a +2 to damage, attack and AC.
• Negate Energy: Psionics + MND check—DC equal amount of damage taken. By taking the standard 2 points of damage, the character can negate the damage from one energy source, including kinetic engergy, by shunting the energy outside of the material universe and into the Outer Darkness. Unlike with other psionic powers, the character can Negate as many attacks per round as he has HP to spend on activating the power without any penalty for multiple targets. It also can be done passively when it is not the character's turn, but it does cost a combat action to use. If used passively in this manner, when the character gets his next turn, he can still move, but is considered to have already spent his other action on this psionic power. Obviously, this is only desirable to do if the damage is higher than the 2 points cost to use the power, and the player does not mind giving up his next action. This can also be used outside of combat to walk through fire, or an irradiated room, or something like that.  
• Dark energy blast: The Knight causes energy directly from Outer Darkness to manifest in regular space and shoot from his hand to blast enemies.  Psionics + MND vs a DEX + Physical to dodge the attack, or a Psionics + MND to oppose, absorb or block the attack (for example, on your psionic shield) The target of a successful check takes 1d6 damage per level of the attacker.
• EMP blast: Psionics + MND vs. Physical + DEX or STR (whichever is better.) Treat this psionic power as if it were an EMP attack; robot and other electrical targets which do not make their save are shut off.
• Psionic suggestion: Psionics + MND vs. Psionics + MND (even characters that cannot use psionic powers have a psionic skill ranking that they can use to oppose suggestions.) A simple opposed check in which the character can make a suggestion seem amazingly reasonable via telepathy. The GM may impose, as with any skill check, situational modifiers that may apply if the target thinks the suggestion is outrageous or over-the-top.
• Clairvoyant sense: Often a Psionics + MND against a static DC set by the GM to see a person or place in the past or present, or even glimpses of possible futures. The GM can also roll checks against this out of combat in secret to give the player clues or hints about things going on around them (such as sensing an old master or enemy nearby.)
• Telepathy: Psionics + MND against a static DC set by the GM to implant a message without speaking (and possibly across great distance—although the speed of thought is not faster than the speed of light). The message is usually short and fairly simple, and comes with no compulsion to act on it. Receptive minds may grant a circumstance bonus to the check while resistant or unwilling minds may cause a penalty, at the GM's discretion.

Using psionic abilities outside of combat is a simple skill check, and since you're outside of combat the hp cost is usually irrelevant and therefore not applied, although NOTE: this does not mean you can use psionics without consequence on something like Prescience right before a combat starts. Don't try to be a rules-lawyer, the GM is the final say, and I can't imagine a GM that would think that's OK. If you're in the midst of a tense scene of some kind, although not combat per se, your GM may rule that you take the hp damage regardless, because it may become very relevant if you are at risk of being involved in combat soon.

Static DCs are meant to reflect d20 standard DCs, in which a DC of 15 is a reasonably "average" task; difficult and requiring some degree of expertise, but should not be beyond even the capabilities of most lower level characters. Anything in the single digits is almost not worth rolling (unless the consequences of failure are suitably dramatic) and anything around 25 or more is a fairly epic task that only higher level characters should feel confident that they can accomplish.

Design notes:  At the bargain price of 2 HP per power use, Knights and other psionic characters start out somewhat weak but become more powerful as levels progress. But, because I cap the game at 10th level, the real runaway psionic prodigies similar to what you may see in some fictional source material may not really be creatable without breaking the rules and going higher level, or perhaps giving them a unique Affinity to using the Force that reduces their HP cost. I don't recommend that for player characters, unless you desire a campaign in which Knights and wizards are much more dominant. The rules as written are meant to maintain a more equal balance between the classes.