(I realize there is an existing Purple Worm started, but it was started 8 years ago ,and was never finished, I am gonna be more active in the coming weeks, as I'm starting a campaign soon that will require some things that aren't here. So this is my attempt at a Purple Worm.)
Purple Worm/Great Worm 
The Chassis for this is the Purple Worm, it's a rather plain and straight forward creature however, so I am utilizing some ideals taken from the "Great Worm" advancement found in Pathfinder. ( Here
https://www.d20pfsrd.com/bestiary/monster-listings/magical-beasts/purple-worm/ ) Anytime I say Great Worm, or Purple Worm, it is the same base creature.
HD:d10
Level Bab Fort Ref Will Feature1 +0 +2 +0 +0 Great Worm Body, Improved Grab, Tremorsense, Con+1
2 +1 +3 +0 +0 Great Worm Path, Stinger, Con+1, Str+1
3 +2 +3 +1 +1 Swallow Whole, Improved Tremorsense,Con+1
4 +3 +4 +1 +1 Worm Path, Toughness, Con+1, Str+1
5 +3 +4 +1 +1 Fast Healing, Worm Anatomy, Growth, Con+1
6 +4 +5 +2 +2 Worm Path, Hard Scales, Con+1, Str+1
7 +5 +5 +2 +2 Superior Tremorsense, Powerful Build, Con+1
8 +6 +6 +2 +2 Worm Path, Improved Toughness, Con+1, Str+1
9 +6 +6 +3 +3 Improved Worm Anatomy, Con+1
10 +7 +7 +3 +3 Worm Path, Mighty Build, Growth, Con+1, Str+1
Skills:4+Int per level (x4 At 1st Level.) Balance, Climb, Intimidate, Knowledge (Nature), Listen, Search, Spot, Survival, Swim
Proficiencies: The Great Worm is proficient in any natural weapon it possesses.
Features :
Great Worm Body : Loses all other racial bonus, and gains Magical Beast Traits (Basically Darkvision 60 feet, and low-light vision.) It's a medium sized Magical beast with a base speed of 20 ft, a burrow speed of 20 ft, and a swim speed of 10ft, One bite attack dealing 1d6+ Str mod (For a medium creature.) They have no limbs capable of fine manipulation, however they count as having two neck slots, and two body slots where beneficial. (An increase in price is expected due to the odd size) Great Worms begin play knowing Common, and Undercommon.
In addition it gains a 1+Natural armor equal to it's Con modifier. Everytime the Great Worm grows a size from Great Worm levels, it gains an extra +1 Nat Armor.
Ability Increases : The Great worm gains 1 Con every level, and 1 Str every even level. To a total of 10 Con, and 5 Str at level 10.
Improved Grab (Ex) : To use this ability, a great worm must hit with it's bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can attempt to swallow the foe the following round.
Tremorsense : The great worm has tremorsense equal to 30ft. At 3rd level this improves to 90ft, and finally at 7th level, this improves to 120 feet, and overcomes special abilities such as Darkstalker, that would otherwise allow a creature to hide from the Great Worm. (They are still allowed a Hide/MoveSilently check as normal.)
Stinger : At 2nd level a Great Worm gains a Stinger natural attack, this is a secondary natural attack dealing 1d6+ 1/2 Str mod (For a medium creature.)
Great Worm Path : At 2nd level, the Great Worm chooses to either evolve towards a natural poison based creature, or into a supernatural elemental creature. Either way this choice is final and alters how all future abilities work. Furthermore, a Great Worm is immune to all of it's own attacks, and abilities, both it's own poison, and in the case of an elemental Great Worm it's own energy damage.
A poisonous Great Worm, gains a poison attack on it's Stinger, it either deals Str or Dex damage (Chosen at 2nd level, this choice cannot be undone or altered after chosen.) A creature damaged by the Great Worm's Stinger receive a poison, inflicting 1d3 damage, Fort save DC 10+1/2HD+Con mod negates. For every 4 HD after 2nd level the Great Worm has, increase the poison damage die by one step. (6,10 etc) At 10 HD, even creatures immune to poison will be affected by this, but they gain a +5 bonus on their saves.
An elemental Great Worm, gains Energy Resistance to two energy types of their choice, equal to 5+5 per 4 HD, or immunity to a single Energy of their choice. (Chosen from Fire, Acid, Electricity or Cold.) In addition to the normal damage from their Stinger, they deal an addition 1d6 points of Energy damage (If they have resistance to two energy types, they choose the type they strike with, on each attack. This is true for all future abilities as well, they may choose the damage type from their resistances for every Great Worm ability.) for every 3 HD they possess.
At 4th Level and every 2 levels there after, they choose another ability to gain from those listed below.
Great Worm Path Abilities:
Worm Path: The choices available to you, depend on whether you are a Poisonous Great Worm, or an Elemental One. Choices for Poisonous will be marked as (Pois) choices for elemental will be marked as (Elem) choices with no markings, can be taken by either.
Bile Blast : The Great Worm may spew out bile from it's stomach, this covers a cone of 10ft +5ft per 2 HD for a Poisonous variant, it simply deals Acid damage, for an Elemental variant it deals damage equivalent to whatever your Energy resistance is. It deals 1d6 Damage +1d6 Per 4HD damage, it has a 3 round cooldown. Reflex Save for half DC 10+1/2HD+Con. This counts as a Breath Weapon for the purpose of Feats.
(Elem) Bile Beam- Requires: Bile Blast : The Great Worm projects it's energy into a solid beam, changing the shape of it's blast from a cone, to a line, which has an effect equal to 15ft +5ft per HD.
(Pois) Bile Cloud- Requires Bile Blast : Instead of acidic bile, the Great Worm may instead vomit an almost fog-like substance that covers an area, this cloud lasts for a number of rounds equal to it's Con Mod, it deals no damage, instead creatures inside must make a Reflex save, or suffer the effects of the Great Worm's Stinger poison. Any creatures moving through the area must pass a Fort save with the same DC as the poison, or become poisoned themselves. Any creature ending their turn in the cloud, is poisoned. Creatures who do not breathe, are immune to this.
Living Ammunition- Requires Bile Blast : If a Great Worm has another creature in it's stomach from the Swallow Whole ability, it may choose as a Swift Action, to vomit them out when it uses it's Bile Blast (Bile Blast is still a standard action.) this is a ranged touch attack, that deals 1d8 bludgeoning damage to both the target creature, as well as the creature that was thrown up, this is in addition to the damage from Bile Blast.(The creature that was thrown up takes this damage even if the attack misses, as they hit the ground). The Range increment is dependent on the size of the creature, as well as the Great Worm. A Great Worm can vomit a creature one size smaller than itself 30 ft, this increases by 30 feet for every size category smaller the creature being thrown up is. (For instance a Medium Great Worm can vomit a Tiny creature 60 feet.)
Purge- Requires Bile Blast, Living Ammunition : If a Great Worm has at least 2 creatures in it's stomach from the Swallow Whole ability, it may as a Full-Round Action, vomit all of these creatures out as part of a Bile Blast, roll a Ranged Touch Attack against the target creature for each creature throw up in this fashion, it deals 1d8 Bludgeoning Damage, to the Target Creature, as well as the creature thrown up (The creature that was thrown up takes this damage even if the attack misses, as they hit the ground). This has the same Range Increment as Living Ammunition.
Cavernous Mouth : This enhances the Swallow Whole ability, the Great Worm's jaw becomes unhinged, allowing it to swallow, and hold a creature equal to it's own size. (But only one such creature fills it entirely) (The Range increment of such a creature is 15ft, for the purpose of other Great Worm Path abilities.)
Subterranean Beast : The Great Worm's burrow speed now scales at 20ft +5ft per HD, in addition to this, it's Darkvision extends out to 120 feet.
Devil's Worm- Requires Subterranean Beast, 8HD : When in an underground environment, it is affected as though it were under the Devil's Sight Warlock invocation, except it has no range. This allows the Great Worm to see in all darkness, both normal, as well as magical, as if it were perfect daylight. Furthermore, it's treated as having Blind Sight out to a range half of what it's Tremorsense is. (Rounded down)
Serpent Beast : The Great Worm's swim speed now scales at 20ft +5ft per HD, in addition to this when underwater it's Darkvision extends out to 120 feet.
Beast of the Deep- Requires Serpent Beast, 8HD : The Great Worm is immune to all pressure effects due to water, or depth and no longer need to breath when underwater, in addition the range increment of Bile Blast as well as it's variants, are doubled when fighting underwater.
(Pois) Deadly Secretion : As a Swift Action, the Great Worm may coat it's body in a slime, that carries the same poison that it's Stinger does, any creature that strikes it in melee must make a Fort Save or be poisoned, any creature Swallowed Whole, is automatically poisoned and must make a Fortitude Save against the Poison. Doing this is tiring, as such it can only be maintained a number of rounds per day, equal to the Great Worm's Con Mod, these rounds can be broken up however it wishes, but if it stops the Secretion, it must spend another Swift Action to activate it again.
(Pois) Toxic Fumes- Requires Deadly Secretion, 8HD : Deadly Secretion becomes an Immediate, or Swift Action, in addition to this, the Great Worm can spend an extra round of it's Deadly Secretion time, to create a cloud centered on itself, that inflicts it's poison to those who linger in it, or stay inside it, Fortitude save negates. The cloud moves with the Great Worm, and lingers for 1 round before dissipating in any squares it moves through. Toxic Fumes share a resource pool with Deadly Secretion, both pooling from the same Con mod pool.
(Elem) Energy Nimbus : The Great Worm may, as a swift action, choose to lower it's Energy Resistance to 1 point or change it's Immunity into Energy Resistance 1, in doing so, chaotic Energy equal to the Resistance it lowered, forms around the Great Worm, any creature moving through an adjacent square takes 1d4 +1d4 per 3HD of energy damage, Reflex for half, DC 10 +1/2HD+ Con Mod. If the Great Worm chose two Energy Resistances, and lowers both to 1, the damage of it's Nimbus deals half, and half. (So, for instance, if the Energy Resistance it chose was Fire, and Acid, it'd deal Half Fire Damage, and Half Acid damage.) This requires a Swift Action to maintain, but can be maintained as long as the Great Worm wishes to do so.
(Elem) Reactive Organs- Requires Energy Nimbus, 8HD: If the Great Worm takes damage from a source of energy damage, with the same type as it normally has immunity or resistance to, while Energy Nimbus is activated, it gains Regeneration equal to it's Con Mod, for two rounds.
Bone Segments : The Great Worms Natural Armor increases by +1 per 2HD, in addition spikes grow out of it's body, these spikes are treated as armor spikes for all purposes, including the ability to be enchanted as if they were a singular weapon. In addition they count as a Manufactured, or Natural Weapon, whichever is more beneficial to the Great Worm.
Spike Burst- Requires Bone Segments : If the Great Worm takes damage, in can, as an immediate action, shoot it's armored spikes off, this is a Burst effect centered on the Great Worm, with an area of 5 ft +5ft per size category it is, larger than Medium. It deals 1d10 Piercing Damage. If enchanted, it deals bonus damage as if it were an enchanted weapon. (So +1 Bone Spikes would deal an extra 1 damage, Flaming Bone Spikes would deal 1d6 Fire Damage as well, etc.) It takes 3 rounds for the Spikes to grow back, however they grow back with any enchantments they may have had.
: A great worm can try to swallow a grabbed opponent of a smaller size than itself by making a successful grapple check. Once Inside, the opponent takes 1d6+Str Mod crushing damage (This scales with Size, so a large creature would deal 1d8+Str and so on.) in addition to acid damage equal to the Great worm's HD every round. (An elemental Worm may instead choose to deal damage of a type equal to a resistance they have.) A swallowed creature can cut it's way out by using a light slashing or piercing weapon to deal 25 points of damage to the gizzard (AC 17). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut it's own way out. (A huge worm's interior can hold 2 large, 8 medium, 32 small, 128 tiny, or 512 diminutive or smaller opponents, Medium can hold 2 small, 8 tiny, or 32 diminutive or smaller opponents.)
Toughness : At 4th level, you gain the Toughness Feat for free, if you already have the Toughness feat you instead gain another feat your meet the requirements for.
Fast Healing : At 5th level you've suffered a thousand small cuts, and have evolved to overcome, you gain Fast Healing equal to Half your HD.
Worm Anatomy : At 5th level your vitals are difficult to pinpoint, you gain 25% fortification, stacking with other forms of fortification. At 9th level this improves to 50% fortification.
Growth : At 5th level the Great Worm grows to large size, it's size increases again at 10th and every 5 HD after. (But only if all levels of Great Worm are taken.)
It's AC, bonus to hit, base damage, grapple and skills change accordingly, but it doesn't get any ability score bonus or penalties.
Hard Scales : At 6th level, After suffering a thousand cuts more, your scales have hardened, you gain DR/Magic equal to your HD, this does not apply to creatures you've swallowed.
Powerful Build : At 7th level, a Great Worm gains Powerful Build. The physical stature of Great Worms let's them function in many ways as if they were one size category larger. Whenever the Great Worm is subject to a size modifier or special size modifier for an opposed check (such as during grapple checks, bull rush attempts, and trip attempts), the Great Worm is treated as one size larger if doing so is advantageous to it. A Great Worm is also considered to be one size larger when determining whether a creature’s special attacks based on size (such as improved grab or swallow whole) can affect it. A Great Worm can use weapons designed for a creature one size larger without penalty(If you can figure out how to wield a weapon without limbs I guess?). However, it's space and reach remain those of a creature of it's actual size. The benefits of this trait stack with the effects of powers, abilities, and spells that change the subject’s size category.
Mighty Build : This functions as Powerful Build, except you always count as your original size whenever beneficial, regardless of any effects that would otherwise change your size. (For instance, if something hits you with a Reduce Person spell, or similar effect you still count as your normal size. If you later take a class that let's you change shape or size, you always count as your original/largest size when beneficial.
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