The way of the four elements is already present in the core book of Dungeons & Dragons 5e, but since I think that the sublcass is way underpowered I made a custom version of it, focussed on a single element instead of all four of them.
Way of Elements
You follow a monastic tradition that teaches you to harness the elements. When you focus your ki, you can align yourself with the forces of creation and bend the four elements to your will, using them as an extension of your body. All the members of this tradition dedicate themselves to a single element, while weaving the elements together is quite often impossible. Even with tireless practice, bending even two elements can’t be done, and monks that have bended all four elements lost their existence in legends.
Way of the Four Elements Spellcasting Table
| Level | Cantrips Known | Spell Known | Maximum Spell Level | Maximum Ki Points for a Spell |
| 3rd | 2 | 1 | 1st | – |
| 4th | 2 | 1 | 1st | – |
| 5th | 2 | 2 | 1st | – |
| 6th | 2 | 2 | 1st | 2 |
| 7th | 2 | 3 | 2nd | 2 |
| 8th | 2 | 3 | 2nd | 2 |
| 9th | 2 | 4 | 2nd | 2 |
| 10th | 3 | 4 | 2nd | 3 |
| 11th | 3 | 5 | 2nd | 3 |
| 12th | 3 | 5 | 2nd | 3 |
| 13th | 3 | 6 | 3rd | 3 |
| 14th | 3 | 6 | 3rd | 4 |
| 15th | 3 | 7 | 3rd | 4 |
| 16th | 3 | 7 | 3rd | 4 |
| 17th | 3 | 8 | 3rd | 4 |
| 18th | 3 | 8 | 3rd | 5 |
| 19th | 3 | 9 | 4th | 5 |
| 20th | 3 | 9 | 4th | 5 |
Subclass Features
Following the way of the Four Elements, you gain the following features at levels 3rd, 6th, 11th and 17th.
Spellcasting
When you choose this way at 3rd level you tap yourself in the elemental disciplines that cross with the arcane. Choose an element from air, earth, fire or water. You learn two cantrips and one level 1st spell of your choice from that element’s spell list. You use your Wisdom modifier as the spellcasting ability for those spells (see chapter 10 of the Player’s Handbook for the general rules for spellcasting).
You learn additional spells as reported in the Way of the Four Elements Spellcasting table. The maximum level of a spell you can learn at each monk level is also reported in the same table (you can learn 2nd level spells at level 7th, 3rd level spells at level 13th, and 4th level spell when you reach 19th level).
Casting Elemental Spells. On the contrary of other spellcasters, you cast the spells you know with your ki points. To cast a spell, you need to expend a number of ki points equal to that spell’s level. You still use its casting time and other rules, but you don’t need to provide material components for it. Casting a cantrip cost you no ki points.
Air
| Spell Level | Spells |
| 0th (Cantrips) | gust, lightning lure, prestidigitation, shocking grasps, thunderclap |
| 1st | expeditios retreat, feather fall, thunderwave, zephyr strike |
| 2nd | gust of wind, levitate, shatter, warding wind |
| 3rd | call lightning, lightning bolt, wind wall, thunder step |
| 4th | freedom of movement, storm sphere |
EARTH
| Spell Level | Spells |
| 0th (Cantrips) | acid splash, druidcraft, mold earth, primal savagery, thorn whip |
| 1st | catapult, earth tremor, entangle, longstrider |
| 2nd | earthbind, dust devil, maximumlian’s earthedn grasps, pass without trace |
| 3rd | meld into stone, erupting earth, slow, wall of sand |
| 4th | stone shape, stoneskin |
FIRE
| Spell Level | Spells |
| 0th (Cantrips) | control flames, create bonfire, dancing lights, fire bolt, produce flame |
| 1st | burning hands, chaos bolt, color spry, hellish rebuke |
| 2nd | aganazzar’s scorcher, flaming sphere, pyrotechnics, scorching ray |
| 3rd | ashardalon’s stride, fireball, daylight, melf’s minute meteors |
| 4th | sickening radiance, wall of fire |
WATER
| Spell Level | Spells |
| 0th (Cantrips) | forstbite, minor illusion, ray of frost, shape water, spare the dying |
| 1st | create or destroy water, fog cloud, frost fingers, ice knife |
| 2nd | blur, moonbeam, rime’s binding ice, snillok’s snowball swarm |
| 3rd | sleet storm, tidal wave, wall of water, water breathing |
| 4th | control water, ice storm |
Martial Spells
Starting at level 3rd when you chose this way, whenever you use your action to cast a spell or cantrip, that action counts as an attack action for the purpose of your Martial Arts and Furry of Blows features.
Elemental Discipline
At 6th level, you can spend additional ki points to increase the level of an spell that you cast, provided that the spell has an enhanced effect at a higher level, as Burning Hands does. The spell’s level increases by 1 for each additional ki point you spend. For example, if you are a 6th-level monk and you cast Burning Hands, you can spend 2 ki points to cast it as a 2nd-level spell (the spell’s base cost of 1 ki points plus 1).
The maximum number of ki points you can spend to cast a spell in this way (including its base ki point cost and any additional ki points you spend to increase its level) is determined by your monk level, as shown in the Way of the Four Elements Spellcasting table.
Elemental Demise
At 11th level you have learn on how to protect yourself from the primal forces of nature. You learn the Absorb Elements spell as part of your spellcasting trait.
It follows the same rules of your other spells, with the exception that you can cast it at its lowest level without expending a Ki point if you use it in response to a type of damage linked with the element you chose with your spellcasting trait: thunder or lightning damage if you chose air, acid damage if you chose earth, fire damage if you chose fire, and cold damage if you chose water.
Fist of Nature
At 17th level you reach the pinnacle of the elemental mastery over a specific natural element. You gain one of the following features, according to the element you chose at 3rd level as part of your spellcasting trait.
Air. At the start of each of your turns you are subjected to the effect of the Thunderous Smite spell as a 2nd level spell until the start of your next turn. You doesn’t need to maintain concentration over it.
Fire. You are permanently under the effect of the Fire Shield spell as a 4th level spell (warm shield version). You can remove or active this feature again on your turn at any time (no action required).
Earth. At the start of each of your turns, if you are not incapacitated, you may choose to gain either a +4 bonus to your AC, or a +4 bonus to each weapon damage roll you make until the start of your next turn.
Water. At the start of each of your turns you are subjected to the effect of the Armor of Agathys spell as a 2nd level spell until the start of your next turn.