Leviathan Root

A creature known as a Leviathan Root, a shapeshifting and sentient plant used to create Magic staves, has merged itself with your flesh.

You can choose whether this bond was of your consent, or of a hungry and Juvenile Root attaching to you in desperation.

due to how rare Leviathan Roots are, Rogues of this Archetype are incredibly few and far between, with most being adventurers and assassins to fulfil the Root's ever increasing desire for liquid and mineral intake.

Leviathan Roots are not native to the Planes, existing in the cliffsides of other worlds and drinking on the ocean and eating the cliff face itself. very little is known of their reproductive cycle, so the only real way you could have gotten one was by micropropagation, growing and acting much different in parasitic captivity compared to their wild, rock-munching counterparts.

Level 3: Minor Shapeshifting/Juvenile Root

at 3rd level, you have a Juvenile Leviathan Root attached to a limb of your choosing. when not attacking, the root pokes above your skin with a draconic looking head. the Root has normal senses with a distance identical to your own, and has tremorsense of 30ft + it's extending range when touching or embedded in stone. the Root can extend from the limb at a distance of 5 ft + double your Dexterity modifier, and can make an Unarmed Attack at a range of 5ft + Your Dexterity modifier.

The Root can do everything a normal appendage can, (e.g Picking up and setting down a Tiny Object, handling a door, holding a weapon, hanging from a ledge, etc), and any Unarmed Attacks it makes deal Slashing damage using your Dexterity modifier instead of your strength. as the Root can hold weapons, any Melee Weapon Attacks using the rooted hand will have an extended range of 5ft + your Dexterity modifier. if you have Spellcasting abilities (such as through multi-classing), the Root can act as a Spellcasting Focus.

The root can reshape itself to the shape of a Tiny object.

Upon getting this Subclass Feature, and every Short Rest after that, Roll a Constitution saving throw at a DC of your rogue level. if you succeed, nothing happens, but if you fail, the Leviathan root drains you of your blood and bone minerals, taking damage equal to your rogue level.

Level 9: Petrovorous Root

The Leviathan Root has learned to harness and consume the latent power of Gemstones to help satiate it's hunger.
When you pick up or carry a gemstone costing 50gp or more, you can choose to attach it to the Leviathan Root, giving it these effects:

The Leviathan Root becomes more docile due to the Gem, making the Constitution saving throw at the end of every Short Rest have a DC of half your rogue level instead of 1x your rogue level.

The Root can change it's damage type to one befitting the Gem that is attached to it, (E.G Force for Rubies, Lightning for Emeralds, Radiant for Diamonds, etc. exact details for every single Gem is up to the DM).

The Root can Cast a Spell adjacent to the attached Gem. E.g Rubies can cast Heal Wounds. (the exact details for every spell a specific Gem grants access to is up to the DM.)
The level of the Spell cast depends on both the age of the Leviathan Root (Represented by your rogue level) and the price of the Gem. (1/4th of your rogue level rounded down from 50gp-500gp, 1/3rd of your rogue level from 501gp-1500gp, and half your rogue level from 1500+gp, E.G if a 10th level rogue has a Ruby costing 1600gp, they can cast Heal Wounds at 5th level.)
you can cast these spells once per Short rest.

Level 13: Major Shapeshifting/Adolescent Root

The Leviathan Root has grown from a Juvenile to an Adolescent, spreading itself around your body and giving you these effects:
You now have the Root's 30ft Tremorsense on stone surfaces due to the Root being just under your skin at any given point.
Even without using the Root, it pokes through your skin on points throughout your body. this gives the Root a 360 degrees field of Vision, given it is not being covered up.
The Root can now be sent forth from any of your limbs, making them all mechanically rooted. 
The root can now shapeshift to form Small constructs as well as Tiny ones

as an Action, you can spread the Root along your entire body, transforming you and giving you these effects based on the transformation:

Wyrm: The Root sprouts wings on your back, giving you a flight speed equal to your walking speed.
Drake: The Root pumps up your limbs with wood, making you faster and more agile. Walking speed is increased by 15 ft, and your AC increases by your Proficiency Bonus.
Wyvern: The Leviathan Root sprouts a giant tail-esque root on your back, and wooden blades shoot out of your arms. The tail functions like a normal Leviathan Root tentacle, and can make an Unarmed Attack as a Free Action. Additionally, all Unarmed Attacks deal an extra 2d8 slashing damage.
Loong: The Leviathan Root grows a breathing Helmet around your head, and fins on your limbs. You gain a Swimming speed equal to twice your Walking speed.

you can transform an amount of times equal to your Constitution modifier (minimum of one), and all uses reset on a Long Rest

Level 17: Adult Root

your Leviathan Root has reached it's full maturity, giving it these benefits;

The Leviathan Root is spikier, giving all hits with it an extra 1d10 slashing damage, this damage also gets applied if you get hit with an unarmed strike while the opponent isn't wearing protection.

The Root's ability to harness gems has enhanced, giving it two uses of the Gemstone's spell instead of one.

The Root is capable of reproducing via the same process of micro-propagation that you got it from, allowing you to make weapons and the such for teammates.

The Root is fully symbiotic with you, meaning you no longer take damage after every Short rest, but it is inseparable from you also.