The recent Unearthed Arcana article of new monastic traditions, which I examined in detail over in Tribality, was the kick in the pants I needed to work on developing another of my own. The monk's meditative nature makes it a good fit for a subclass about dreams and astral travel, much as the 2e psionicist does - it's not for nothing that the 4e monk is psionic, after all. Which brings me to this, the Way of the Silver Chain - monks, but also knights-errant in dreams and in the Astral Plane.
Way of the Silver Chain (Monk)
The monks of the Way of the Silver Chain draw upon the power of dreams and the Astral Plane to protect the waking and material world. Their name comes from the power of their ki that strengthens their silver cords against attack. They are among the rarest and most esoteric of monastic traditions, but they aid travelers who need things far beyond this world.
Dreaming Warrior Technique
When you choose this monastic order at 3rd level, your ki becomes inextricably linked to dream, nightmare, and the Astral Plane. When you hit a creature with one of the attacks granted by your Flurry of Blows, you can impose one of the following effects on the target. You may not impose more than one condition per turn.
It must succeed a Wisdom saving throw or become frightened until the end of your next turn.
If it is currently frightened, your weapon attacks deal an additional 1d4 psychic damage to it.
It must succeed a Charisma saving throw or become vulnerable to psychic damage until the end of your next turn. Creatures with resistance to psychic damage instead take normal (not double) damage from psychic sources. Creatures with immunity to psychic damage remain immune.
For the next minute, it takes psychic damage equal to your monk level whenever it makes an opportunity attack.
Strength of the Silver Chain
Also at 3rd level, when you travel in the Astral Plane and an attack would damage your silver cord, you can choose to expend all of your remaining ki points to reduce the damage to 0. You may not use this feature if you currently have 0 ki points.
Starting at 6th level, as part of any long rest, you grant powerful, strengthening dreams to up to five allies who also completed a long rest in the same camp, inn, or other building. The target creatures' current and maximum hit points increase by 5 until they begin a long rest. This cannot effect creatures that do not dream. At 11th level, current and maximum hit points increase by 10 instead of 5. At 16th level, current and maximum hit points increase by 15.
Further, you may cast the non-hostile version of dream without expending a spell slot. Once you do so, you must complete a long rest before using this feature again.
Starting at 11th level, you gain resistance to psychic damage. As a reaction, when you or your allies take psychic damage, you can spend 1 ki point per creature suffering psychic damage. A creature you spend ki to protect gains resistance to psychic damage until the end of their next turn. If they already possess resistance to psychic damage, they reduce psychic damage from that source to 0.
Also at 11th level, when you travel in the Astral Plane, you automatically succeed Intelligence saving throws against psychic wind. When you do so, you may either regain all of your ki points, or you may spend 2 ki points to convert an ally's failed Intelligence save against psychic wind to a success (up to the limit of your ki pool). Further, you gain advantage on Charisma checks to interact with githyanki and other creatures native to the Astral Plane.
Grandmaster of Dreams
Starting at 17th level, when you are adjacent to any unconscious or frightened creature and have at least half of your speed remaining for the turn, you may expend the rest of your speed for the turn to teleport to a space adjacent to any other unconscious or frightened creature within 120 feet.
Further, you can cast astral projection for half the normally-expended material cost, without expending a spell slot. Once used, you cannot do so again until you complete a long rest.
Also at 17th level, when you critically hit a creature that has an astral body with a weapon attack, you can choose to deal no damage and instead cut the silvery cord that tethers it to its material body.
As with a lot of specialized character concepts, the Way of the Silver Chain has features that they don't get to use often. I've attempted to address this by giving two features at 3rd and 11th level, so that you're getting something that is usable a lot of the time, and the "while in the Astral Plane" feature is more of a ribbon.
Dreaming Warrior Technique riffs on the Open Hand technique, by giving the monk a list of nasty effects when they hit with Flurry of Blows. I wanted to draw on some of the setup/combo goodness of 4e here - Silver Chain monks can set up their own combos, sure, but they're really rolling if their allies are tossing out the frightened condition as well, and if a lot of psychic damage gets thrown around, even better. Instead of locking out reactions for one round, the monk punishes the most common kind of reaction for 1 minute.
So Dreaming Warrior Technique improves their attacks, Dream Guide is a free aid effect, Iron Will is a defensive boost for a somewhat-rare damage type, and Grandmaster of Dreams is an odd mobility power - Shadow Step's extra-weird cousin. Judging balance is always tricky, but I've given this my best effort.
While in the Astral Plane, your silver cord is hard to cut (don't let the githyanki knight get two crits against you), you can survive or even thrive in the psychic wind (or help your allies get by), and eventually even cut silver cords with your bare hands, as a githyanki knight does. But, you know, it kills the person instantly, so depending on the situation, you might not even want to do that.