Base Class: Monk
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Elemental Strike
Starting when you take this subclass at 3rd level, you gain the ability to imbue your punches, kicks and the like with planar energy. You can expend 1 Ki point to give your unarmed strikes a bonus 1d4 of elemental damage in addition to their existing damage. To determine this damage's type, roll 1d8, and pick the corresponding value from the chart that the spell Chaos Bolt uses for it's damage type. This ability can stack onto the same attack, but you only roll for the damage's type once per strike.
Bond of the Planes
At 6th level, you gain the ability to meditate and gain a buff to one of your monk weapons thanks to the planar gods. Choose a monk weapon that you are proficient with. That weapon now gets an extra 2d8 elemental damage, the type of which is decided by choosing one of the d8's and corresponding that value to the Chaos Bolt table. If the d8's roll the same value, roll another 1d8 and add it to the damage total. This ability requires attunement, and you can change the monk weapon it affects after a long rest
Universal Ward
Starting at 11th level, you gain the ability to cast either Contact Other Plane or Conjure Elemental without the need for material components, and you have advantage on saving throws made to make these spells work (INT for Contact Other Plane to avoid the massive psychic damage and CON for Conjure Elemental to hold your concentration). You can only cast one of these spells per long rest, but you can use this feature again at the expense of 10 Ki points. If you do this, you now have disadvantage on saving throws instead of advantage
Planar Master
Upon reaching 17th level, roll 1d4. This roll cannot be affected by anything at all. Starting at this level, you can cast Demiplane a number of times equal to the dice rolled. Once you run out of uses, this ability becomes dormant for 2 sessions. On the third session, roll another 1d4, again unaffected by any outside modifiers.