Adapted from "la Voie des Marchombres" (literally: The Marchombre's Path), and paying homage to this series of french fantasy books written by Pierre Bottero (therefore the features are French named), this rogue archetype is control and utility-based, not being damage oriented.

Some features (along with the additional ones) would be extremely strong in lots of game settings and so it will need to be balanced by your DM. Ours was leaning heavily on homebrew and high CR world for us and so, when everything is OP, nothing is!

To fully understand the lore behind this subclass, you would need to read the books and I will not be copying material from those or start a free translation here. However there are still key concepts that need to be understood.

A marchombre...

• fights for Harmony and Freedom against Chaos.
• tends to stay away from "good or evil" concepts.
• adhere to their Guild's Pact (namely Le Pacte), stating they should never use their power to dominate over others but only to progress on la Voie des Marchombres.
• trains a lot as they believe their body (and mind) is a vessel they use to go about the world, crossing boundaries and limits, always to progress on la Voie des Marchombres.
• knows that la Voie des Marchombres is infinite, perilous and difficult to progress on and that you do not compare to another when progressing, only to oneself.

It is very basic. The books contain poetry, conversations between master and apprentice, and a generally great story, all explaining and helping get a broader view of the nuance of it all. Yet, those key concepts are a good starting point.

Note: I am no longer playing the campaign, as such I am publishing this only for others to build upon and enjoy!

Spellcasting

The Marchombre Abilities

When you reach 3rd level, you gain the ability to cast "spells", between quotes, as you are not an actual spellcaster but rather develop your marchombre's power (spell-like abilities written as SLA) in order to continue progressing on la Voie des Marchombres. However, do see Spells Rules for the general rules of spellcasting and the Spells Listing for the wizard spell list.

This subclass can use spells from all schools of the Wizard, up to level 4.
However, they are limited only to non-damage dealing spells, therefore leaning heavily toward movement, utility and some control spells.
The only exception to this spell limitation are the SLA you learn at 3rd, 8th, 14th, and 20th level.

Is it so because a marchombre does not use its power to dominate over others, but only to progress on la Voie des Marchombres.

There are a lot of possible options since Wizard have a rather large spell list, so choose those that will suit your role play. Furthermore, finding items/scrolls imbued with spells might widen your potential selection. For example, you might find something that gives you a Feather Fall-like ability, and so you could replace such spells with something else (at level up). Keep in mind to not choose only based on mechanical interest because you need to remember that a marchombre do not use its power to dominate over others. Otherwise, you might get reminded of Le Pacte by your DM with an event in your game...

Cantrips

You learn Mage Hand and two other cantrips of your choice from the wizard spell list. You learn another wizard cantrip of your choice at 10th level.

SLA Slots

The Marchombre Spellcasting table shows how many SLA slots you have to cast your wizard spells of 1st level and higher. To cast one of these SLA, you must expend a slot of the spell’s level or higher. You regain all expended SLA slots when you finish a long rest.

SLA Known of 1st-Level and Higher

The SLA Known column of the Marchombre Spellcasting table shows when you learn more wizard spells of 1st level or higher. Each of these SLAs must be a non-damage dealing spell of your choice, and must be of a level for which you have SLA slots. For instance, when you reach 7th level in this class, you can learn one new SLA of 1st or 2nd level.

Whenever you gain a level in this class, you can replace one of the wizard spells you know with another SLA of your choice from the wizard spell list. The new SLA must be of a level for which you have SLA slots and be a non-damage dealing spell.

Ritual Casting

You can use a Wizard spell as a ritual if that spell has the ritual tag and you know the SLA. If you lean toward a more "Arcane-Focused" character, could take a Ritual Casting (Wizard) Feat to further broaden your spell list, keeping ritual spells in a spell book, conserving your known SLA for other Wizards spells.

Spellcasting Ability

Charisma is your spellcasting ability for your SLA, since you learn your SLA through dedicated meditation, training and clever wording. You use your Charisma whenever a SLA refers to your spellcasting ability. In addition, you use your Charisma modifier when setting the saving throw DC for a wizard spell you use.

Spell save DC = 8 + your proficiency bonus + your Charisma modifier

You do not have a Spell attack modifier, as you do not have any damage dealing spells.

Marchombre Abilities

La Main de l'Ombre

Starting at 3rd level, when you use mage hand, you can make the spectral hand invisible, and you can perform the following additional tasks with it:

• You can stow one object the hand is holding in a container worn or carried by another creature.
• You can retrieve an object in a container worn or carried by another creature.
• You can use thieves’ tools to pick locks and disarm traps at range.
• You can perform one of these tasks without being noticed by a creature if you succeed on a Dexterity (Sleight of Hand) check contested by the creature’s Wisdom (Perception) check.

In addition, you can use the bonus action granted by your Cunning Action to control the hand.

Réflexe du Marchombre

Starting at 3rd level, you add your Spellcasting Ability as a bonus to your initiative rolls. Because of your relentless training, you have gained the ability to move a bit faster in order to control the timing of a situation.

If you decided to go purely martial and do not have a Spellcasting features, you can use your Wisdom modifier instead.

Persuasion du Marchombre

Starting at 9th level, you get a new custom skill (that you need to manually add to your character sheet) called Animal Persuasion (CHA based) that gives you expertise in trying to persuade animals to follow your Murmure Marchombre and what you are requesting through it.

Le Murmure Marchombre

Starting at 9th level, as an action, your dabbling of Le Chant Marchombre enables you to whisper a variation of it when close and personal to your target (5 ft). The target must succeed on a Charisma saving throw against your Spell Save DC or be paralyzed and stunned for 1 minute. At the end of each of its turns, the target can make another Charisma saving throw. On a success, the effects end on the target.

The target has disadvantage if you are not in combat (out of initiative) or to the DM's discretion.
Doing Le Murmure Marchombre does not break your stealth and only your target can notice your whisper.

Once you use this feature, you can’t use it again until you finish a short or long rest, when you complete your Gestuelle Marchombre.

Starting at 10th level, Le Murmure Marchombre can now be used at range (60 ft), using the flow of the wind (Gust-like ability).
Doing so at range might break your stealth and will require a Stealth Check to stay hidden.

Additionally, you gain the ability to merge into Gaseous Form at will out of combat.

Le Chant Marchombre

At 9th level, your mastery of the Murmure Marchombre now enable you to use Le Chant Marchombre (literally, The Marchombre's Song). This strong control ability is able to paralyze in his attack a raging barbarian, stop a spellcaster or pacify wild creatures.

As an action, any creatures you choose up to your Proficiency Bonus within 60 ft must succeed on a Charisma saving throw against your Spell Save DC or be paralyzed and stunned for 1 minute. At the end of each of its turns, the target can make another Charisma saving throw. On a success, the effect ends on the target.

Once you use this feature, you can’t use it again until you finish a short or long rest, when you complete your Gestuelle Marchombre.

Tridextrie de l'Ombre

Starting at 13th level, your mastery of your Main de l’Ombre becomes apparent, and as you find more ways to target vulnerabilities on enemies, your attacks become deadlier.

Your Main de l’Ombre is now permanently present and counts as one of your hands, carrying whatever it can or doing a melee attack. Therefore, you can now use a bonus action granted by your Cunning Action to make a Dexterity (Sleight of Hand) check, use your thieves' tools to disarm a trap or open a lock, take the Utilize action, or take the Attack action.

You can add your ability modifier to your Off-hand damage or your Main de l’Ombre attack. The Main de l’Ombre does not get to make an opportunity attack.

Furthermore, you also gain an additional way to use your Sneak Attack: you can apply your Sneak Attack damage on your melee attacks, even if you have disadvantage. All the other rules for Sneak Attack still apply to you. You may also use your Main de l’Ombre to flank enemies with other allies, gaining flanking bonuses on your Main de l’Ombre’s attack.

Whenever you deal Sneak Attack damage, you may choose an effect (damage does not scale with level) to apply immediately to your target without a saving throw from it (ie. Bane before other checks), lasting until the end of your next turn.

• Bane - Subtract 1d4 AtkRoll/Saving Throws
• Shocking Grasp - No reaction, +1d8 dmg
• Chill Touch - No healing, +1d8 dmg
• Ray of Frost - Subtract 10ft movement, +1d8 dmg

Chevaucheur de Brume

Starting at 17th level, you learn and train the ability to ride the mist to an even greater degree.

If you did a successful Sneak Attack on your turn, you can vanish at the end of your turn from your current plane of existence (similar to Blink) and appear in the Ethereal Plane (the effect fails if you are already on that plane). At the start of your next turn (after choosing to use your Bonus Action or not) or if you choose to before that, if you are on the Ethereal Plane, you return to an unoccupied space of your choice that you can see within 10 feet of the space you vanished from. If no unoccupied space is available within that range, you appear in the nearest unoccupied space (chosen at random if more than one space is equally near).

Before reappearing, you can choose to use your bonus action to make a Dexterity (Stealth) check to be considered hidden upon exiting the Ethereal Plane.
Upon exiting the Ethereal Plane, you gain the ability to use your Sneak Attack nonetheless.

Additionally, you gain the ability to cast Dimension Door once per long rest.

Timeless Body

Starting at 17th level, all the years spent meditating and crossing boundaries enables you to go in between the weaves of time itself, causing you to age more slowly. However, you still need to eat and drink. For every 10 years that pass, your body ages only 1 year.