Ratcatcher Rogue

Ever since the Ashen Age drove people into large enclosed communities, rats have been spreading diseases among the populace. Ratcatching became an important and even lucrative profession, and some clever people developed it into an artform.   Those roguish ratcatchers learned to train the rats they captured to use them in their covert operations which often included pickpocketing and lockpicking, and sometimes even assassinations. Some of them could even tame the menacing cranium rats that plague the great capitals.  

Ratcatcher Features

3rd level: Collection of Rats, Plague Rats

9th level: Rat King, Trained Rats

13th level: Collection of Cranium Rats

17th level: Cranium Rat Mastermind

Collection of Rats

Starting at 3rd level when you choose this archetype, you can spend an activity during a long rest to collect rats from the vicinity of your resting place. You then have a swarm of rats at your disposal hidden in your clothing. You can only ever have one swarm of rats at a time under your control, and the rats disperse if you die.   You can use a bonus action to send out an individual rat or the whole swarm and make it move up to its speed. Once the rat or the swarm is out and shares its space with another creature, you can use your action or your bonus action to command it to use its attack. You can use another bonus action to have the rat or the swarm move again and if it shares your space, hide back into your clothing. If the swarm is killed, you need to use this feature again to collect another.   You also get a new use of your Sneak Attack feature: you get to add sneak attack damage to your attacks as long as the target shares its space with at least one rat that you have collected and you do not have disadvantage on the attack.  

Plague Rats

Also at 3rd level, you can use a vial or potion of poison as a bonus action to coat the swarm of rats you have collected. The swarm itself does not suffer any ill effect from the poison. Each individual rat from that swarm then causes an extra 1d4 poison damage with its bite, while the whole swarm causes an extra 1d6 poison damage. The poison wears off after you finish a long rest.  

Rat King

By 9th level, your trained swarm of rats is hardier than others of its kind. The swarm and individual rats belonging to it gain +1 to their attack rolls and deal extra damage with its bites: 1d4 for individual rats and 1d6 for the whole swarm.  

Trained Rats

Also at 9th level, you can train your collection of rats to be able to utilize thieves' tools and perform other legerdemain. As a bonus action, you can utilize your swarm of rats in new ways:

equip your swarm of rats with thieves' tools and command it to pick locks or disarm traps for you, using your thieves' tools skill modifier.

command the swarm to use your Dexterity (Sleight of Hand) skill for you in applicable situations, such as picking pockets or switching items.

command the swarm to use the Help action to give you or an ally advantage on a skill check or to distract an adjacent enemy to give you or an ally advantage on the next attack against it.

Individual rats cannot perform these skills on their own.  

Collection of Cranium Rats

At 13th level, you learn how to collect a swarm of cranium rats during a long rest activity. These rats have an enlarged brain that gives them psionic capabilities.   You can use this swarm the same way you can use your common swarm of rats, except it can also cast spells if you use an action or a bonus action to command it to.  

Cranium Rat Mastermind

At 17th level, you can collect a cranium rat mastermind during your long rest activity. This swarm can hold a roughly humanoid shape, and if you clothe it or equip it with light armor, it can pass as a humanoid unless a DC 12 Investigation check is made to discern it. Otherwise, you can use it the same way you use your other swarms. It has the following statistics:   Cranium Rat Mastermind

medium swarm of tiny aberrations

typically lawful evil

AC 12 (modified if you equip it with light armor)

HP 98 (20d8)

Speed 30 ft.

STR 10 (+0) DEX 14 (+2) CON 12 (+1) INT 16 (+3) WIS 11 (+0) CHA 14 (+2)

Damage resistances: bludgeoning, piercing, slashing, psychic

Condition immunities: charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned

Senses: darkvision 30 ft., passive Perception 10

Languages: telepathy 60 ft.

Challenge: 7 (2900 xp)

Features:

Illumination (bonus action to shed dim light in 5 ft. radius, increase to bright light up to 20 ft. radius (and dim light for up to additional 20 ft.), or extinguish light)

Innate Spellcasting (psionics, casting ability INT, spell save 14; as long as Mastermind has more than half HP, it can cast spells)

At will: command, comprehend languages, detect thoughts, disguise self, mind sliver, sending

1/day: confusion, dominate monster, telekinesis

Swarm Master (Mastermind can occupy another creature's space and vice versa, and move through openings large enough for a tiny rat. Mastermind can't heal HP normally, but can merge with a Swarm of Cranium Rats to gain all its remaining HP.)

Telepathic Shroud (Mastermind is immune to any effect that would sense emotions or read thoughts, and all divination spells)

Actions:

Bites (melee weapon atk. +5 to hit, reach 0 ft., one target in Mastermind's space. Hit: 18 (6d6) pierce dmg or 9 (3d6) pierce dmg if Mastermind has more than half its HP, plus 26 (6d8) psychic dmg.)

Call the Swarm (recharge 5-6; a new Swarm of Cranium Rats appears on its own initiative turn from a crack in a wall, floor or ceiling. This swarm of cranium rats disperses if the Cranium Rat Mastermind is killed or called back to your belongings.)