Monster Soul

This is my modification of Cowboy_Tiefling's Blue Mage subclass for the Sorcerer.

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Monster Soul Sorcerers are those who carry within themselves a fragment of the primordial power that all manner of dangerous creatures possess, a monster soul.  Whether inherited from their ancestors, absorbed from exposure to a monster at the moment of its death, or received as a gift, these Sorcerers use their soul to fuel their magic and learn to fight like monsters do.

Monster Affinity

The power of your monstrous soul flows through your body.  Your body and your ability to discern the type and capabilities of monsters are enhanced.

At 1st level, your hit point maximum increases by 1 and increases by 1 again whenever you gain a level in this class.

You do not need to provide any material components for spells unless they have an explicit cost in their description.

Your proficiency bonus is doubled for any ability check you make to identify monsters and their special powers or vulnerabilities. If you lack proficiency in the required skill, for the purpose of these monster knowledge checks, you are treated as having proficiency, however, it is not doubled.

In addition, your monstrous soul has altered your body.  You possess a natural weapon which you can use to make unarmed strikes.  You may use your Charisma modifier for attack and damage rolls with this natural weapon.  This class feature does synergize with other natural weapons, but not directly with unarmed strikes.  For example, a Tabaxi monk cannot use this feature with a plain unarmed strike to deal 1d10 + Charisma modifer slashing damage, however, they could use it with their claws to deal 1d4 + Charisma modifier slashing damage.

Choose between: 

A) Claws: 1d4 + your Charisma modifier slashing damage

B) Fangs: 1d4 + your Charisma modifier piercing damage

C) Tail: 1d4 + your Charisma modifier bludgeoning damage

Monster Mimicry

At 1st level, your unique physiology allows you to absorb and permanently learn monster abilities and spells.  When you are subject to a monster's non-weapon ability or spell, you may use your reaction to attempt to learn that ability.

Make a Concentration check. If the ability causes damage, the DC equals 10 or half the damage you take, whichever number is higher. If the ability does not cause damage, the DC equals 8 + the creature's proficiency bonus.

On a success, you may add the ability to your list of Monster Skills. You may only know a number of Monster Skill equal to half your Sorcerer Level (rounded down) + Proficiency Bonus. If learning a new Monster Skill would cause you to exceed this maximum, you may choose a Monster Skill you know and replace it with the new one.

See the table in the Monster Skills section below for details on what level Sorcerer you need to be to learn a Monster's Skill.  You cannot learn lair or legendary abilities.

You can use this ability a number of times equal to your half your proficiency bonus (rounded down) and regain your uses after you finish a short or long rest. 

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A note on intention:  'monster' as a creature type does not exist.  Generally speaking, any creature that isn't a normal humanoid you'd see on the street is considered a monster.  Use your best judgement on whether something would be considered a monster.  Some examples of monsters include: a bear, chimera, dragon, elemental, imp, and a zombie

Monster Skills

You may cast any monster spell or ability from your Monster Skill list as if it were a Sorcerer spell.

Non-spell abilities on your list of Monster Skills must be cast by expending a spell slot of an appropriate level, determined by the Challenge Rating of the creature you learned it from, as noted in the table below. This table should be used as a guideline; the DM may choose to raise or lower the required spell slot level at his or her discretion based on the relative power of the learned ability.

Spell Slot Level by Challenge Rating:

Challenge	Slot Level	Minimum Sorcerer Level
<1	0th (Cantrip)	1
1-2	1st	1
3-4	2nd	3
5-6	3rd	5
7-8	4th	7
9-10	5th	9
11-12	6th	11
13-14	7th	13
15-16	8th	15
>17	9th	17

You do not need to provide any material components for spells or abilities cast in this way unless they have an explicit cost in their description.

Abilities learned this way may be affected as normal by Metamagic once you've gained that feature at 3rd level

Monster Consumption

At 6th level, you gain the ability of Monster Consumption. You can attempt to learn passive or defensive abilities from creatures by eating parts of them. Some Sorcerers eat the creature’s heart, others the brain. The specific part of the creature consumed doesn’t appear to matter, only that the Sorcerer believes the Monster's power is lies within.

Consuming a corpse in this manner takes at least 10 minutes and requires you to spend 1 Sorcery Point. Make a Constitution saving throw, the DC equals 8 + the creature's proficiency bonus + the creature's Constitution modifier. On a success, you learn the ability as if it were gained with Monster Physiology. Otherwise you become sick and vomit up the corpse which is ruined.

You add this ability to your Monster Skills. Each passive ability costs 1 Sorcery point to maintain, reducing the available Sorcery points by that amount. These abilities are always ‘on’ and cannot be turned off to regain access to the reserved Sorcery points. The Sorcerer's form often changes to reflect the abilities learned. For example, if you gain the natural base armor of the Galeb Duhr, then your skin can take on a rocky appearance.

Monster Counter

At 14th level you learn the spell Counterspell. It does not count against your spells known and you cannot swap it out at a later level. If you already know Counterspell, you may learn one new spell to fill that slot.  You may increase the level of this spell by expending 4 Sorcery Points per level.

Choose between:

A) You can cast Counterspell as a 5th-level spell without expending a spell slot.  After using this ability, you may not do so again until you complete a long rest.

B) Your Counterspell can now also interrupt a monster in the process of using an ability.  The calculation for what spell level a monster ability is considered uses the Monster Skill table listed above.

Perfected Monstrosity

At 18th level, though still limited in the number of powers you can have, passive abilities no longer require Sorcery Points to maintain, freeing up those Sorcery points for other uses. Your ability to learn Monster Skills is no longer limited by CR. You may now also learn legendary skills.