Like their cousins the githyanki, the githzerai are a rigidly disciplined people from another plane of existence. Unlike the warlike githyanki, the githzerai apply their discipline to asceticism and self-defense. Essentially, they’re psychic super-monks, bulwarks against chaos.
The githzerai monk has modestly above-average Strength and Constitution and high Dexterity, usually the physical ability contour of a sniper; a melee combatant with this profile generally has to use movement and surprise to compensate for its reduced ability to take damage and dish it out. The Monster Manual’s solution to this problem is curious: Rather than make the githzerai monk a skirmisher with abilities like Nimble Escape, the MM makes it, for all intents and purposes, a brute. That is, its abilities are tailored to toe-to-toe melee fighting.
The githzerai monk’s Psychic Defense feature lets it add its Wisdom modifier to its armor class while unarmored, giving it AC 14. Its Unarmed Strike does 1d8 + 2 bludgeoning damage (!) plus 2d8 psychic damage (!!), and its Multiattack lets it make two such strikes per turn (!!!). This is an order of magnitude beyond what damage any player character is capable of dealing. One blow from the hand of a githzerai monk is like being smacked by a warhammer and subjected to a mini–Mind Blast.
Meanwhile, the githzerai monk can cast shield three times a day, boosting its AC from 14 to 19 as a reaction each time. Its hit points aren’t astronomical, but it can survive a while simply by not letting itself get hit. Key to this strategy, however, is not letting itself be double-teamed: the efficacy of shield drops off drastically if even one additional attacker is aiming blows at it.
The idea of the githzerai monk as a “psychic brute” is interesting, but I’ve gotta say, I’m perplexed as to why the MM would leave it bereft of any skill or feature that would let it function as a skirmisher if it wanted to. It’s not just that the githzerai monk is better at toe-to-toe melee, it’s that it’s only good at toe-to-toe melee. It makes no sense to me. Whatever—if you’re running a githzerai monk, toe-to-toe melee is pretty much how it has to fight. (I almost wonder whether the sole point of the githzerai monk’s powers is to punish foolish players who underestimate them.)
The innate jump spell has the same drawback for the githzerai monk as it has for the githyanki warrior: it costs an action, meaning it can’t be combined with an attack. Feather fall just makes me shake my head and wonder why. Because they all live in mountaintop monasteries, and accidents are common?
If a githzerai monk is fighting you at all, it’s probably got a good enough reason to want to finish the job if it can, but it will Dash away in the fourth round of combat or if it’s seriously wounded (reduced to 15 hp or fewer).
The githzerai zerth is a more advanced version of the monk, with higher Constitution (and thus an even more skirmish-y profile, although it’s just as bereft of skills and features that would allow it to fight as such), even more Unarmed Strike damage, and the spells phantasmal killer and plane shift, which it can use once per day each.
Phantasmal killer is a surprisingly ineffective ability: At the time of casting, it demands a Wisdom saving throw to avoid the frightened condition, then grants another save, against psychic damage, at the start of the target’s turn. Any successful save ends the spell. The spell save DC is 14; figuring an average +3 saving throw modifier, and thus a 50 percent chance of making the save, you can expect a PC to shake this spell off before ever taking any damage. Between its Intelligence 16 and the fact that this is an innate ability, we’ll be kind to the zerth and assume it can made shrewd guesses about which of its enemies have Wisdom save modifiers low enough to make the attempt worthwhile—but honestly, I put “worthwhile” at around −3, which is what it takes to give the zerth about a two-thirds chance of inflicting a single round’s worth of damage. Yeah.
Plane shift, on the other hand, only has to work once for a githzerai zerth to more or less permanently dispense with an enemy it really wants to see gone, and all it needs to feel good about its chances of success is a Charisma modifier of 0 or less. Alternatively—and more likely, if it’s the only zerth in its group—it will keep plane shift in reserve for its own escape, with its allies in tow, if things go awry.
Like the githzerai monk, the zerth is a brute disguised as a skirmisher, and its greatest effectiveness is in toe-to-toe melee against a single opponent. (Its AC is high enough that it might be able to handle two attackers at once, but not three.) When it’s seriously injured (reduced to 33 hp or fewer), it will give an audible signal and Ready (action) the plane shift spell, triggered by its final ally’s joining hands with it. If it has allies who are fleeing, it will fight as a rearguard to cover their escape, making sure they’re free and clear before using plane shift to save itself.