Lord of Rats

While some rogues seek magic, some seek psychic power and some seek martial skills you instead have found kinship among those that people consider Vermin, rats, you speak to them, you use them as eyes and have even bonded with a powerful rat that serves to stand side by side with you on battle and in thievery. 

Level 3: Vermin Bond

You have bonded with a Giant Rat on the spiritual level, it will grow on strength as you do. 

The Rat is Friendly to you and your allies and obeys your commands. It vanishes if you die.

The Rat in Combat. In Combat, the Rat acts during your turn. It can move and use its Reaction on its own, but the only action it takes is the Dodge action unless you take a Bonus Action to command it to take an action in its stat block or some other action. You can also sacrifice one of your attacks when you take the Attack action to command the beast to take the Bite action. If you have the Incapacitated condition, the beast acts on its own and isn't limited to the dodge action.

If the rat dies you may take the magic action to restore it to full HP, you regain your use of this ability on a short rest. 

Giant Rat Spirit

Small Beast

Armor Class 13 + proficiency

Hit Points 5 + five times your rogue level. 

Speed 30 ft

STR

12 (+1)

DEX

16 (+3)

CON

14 (+2)

INT

8 (-1)

WIS

14 (+2)

CHA

6 (-2)

Condition Immunities poisoned

Damage Resistances: poison

Senses darkvision 60 ft., passive Perception 12

Languages understands the languages you speak, also speaks Rat

Challenge — Proficiency Bonus (PB) equals your bonus

Traits:

Keen Senses: The rat has advantage on wisdom (perception) checks related to smell. 

Pack Tactics: The rat has advantage on attack rolls when an ally is within 5ft.

Vermin Bond: The rat adds your proficiency bonus to all saving throws and ability checks it makes. 

Attacks:

Bite. Natural Weapon Attack, your dex + prof to hit, 5ft range, single target. Hit 1d6+2+your dexterity mod piercing damage, target must make a constitution save with a DC of 8 + your dex + proficiency or have the poisoned condition for 1 round. 

Level 3: Spies In The Shadows

You've spent so long around rats you've gained the ability to understands their speech and communicate back to them, you may make charisma checks rather than animal handling checks when speaking to them, additionally due to your experience dealing with rats you have advantage on all influence checks against rats. 

Level 3: Rat Companion

You have a small rat companion that follows you everywhere. 

You gain the ability to cast Find Familiar without spending a spell slot or paying material components once per long rest as long as its only used to summon a rat. 

Level 9: Vermin Resistance

Your time around rats has altered your biology, you gain resistance to poison, immunity to the poisoned condition and disease and advantage on perception checks related to smell. 

Level 9: Poisoned Strike

Spending time around rats has had you in close proximity with many disgusting and deadly things, now it's time to put them to use. 

When making a sneak attack you may deal poison damage instead of piercing damage and ignore resistance to it. 

Level 13: Unrelenting Bites

Your Giant Rat Spirit has grown ferocious. 

When you command your Giant Rat Spirit to take the attack action it may attack twice instead of once. 

Level 9: Magical Bites

Your Giant Rat Spirits teeth have grown strong and their damage now counts as magical piercing. 

Level 17: Plague Bringer

Your time around your rats has granted you even greater power, you may now channel primal magic from them. 

You gain the ability to cast both insect plague (made of rats) and contagion once per long rest.