Level 1:

  Natural Dexterity: In your beast form (hybrid or giant rat), you have a Natural Armor of 11 + Dex Mod. You also gain a Dexterity score of 16 if your score isn't already higher. 

  Speed: In your beast form, you have a walking speed of 40ft. Additionally, you gain a climbing speed of 20ft. 

  Natural Stealth: In your beast form, you have an advantage on Stealth checks while not in direct sunlight. 

  Small Stature: In your Giant Rat form, you can comfortably squeeze into a space as small as 1ft x 1ft. 

   Darkvision: In your Giant Rat form, you gain Darkvision up to 60ft.

  Bite (1d6 + Dex Piercing): In your beast form, your unarmed attacks become bites with which you are proficient. Your bite is considered a finesse weapon. 

  Pack-a-Snack: Due to your small size, you require less raw, fresh meat to sustain your Bloodlust. You are able to feed off of Tiny creatures to sustain yourself. Feeding in this way does not grant you Temporary Hit Points. 

Level 3:

   Venomous Fang (Save DC = 8 + PB + DEX): In your beast form, your body naturally begins to produce a powerful toxin. When you hit a creature with your bite attack, you can choose to have them make a Constitution saving throw or become Poisoned for 1d4 rounds. You can use this ability a number of times equal to your Constitution modifier and regain all uses when you feed. 

Level 7:

   Resilience: While in your beast form, you gain resistance to slashing, piercing, and bludgeoning damage from non-magical, non-silvered weapons. 

   Improved Natural Weapons: Your bite attacks are now considered magical for the purpose of overcoming magical resistances. 

   Improved Dexterity: As a Wererat, you're more able to use equipment designed for humanoid creatures. In your beast form, you can use Simple Melee Weapons, which lack the heavy or two-handed properties, and Light Crossbows. 

Level 11: 

   Ambush Tactics: Your small stature and natural quickness allow you to weave into and out of combat. When you make a melee attack against a creature in your beast form, if there is another ally within 5ft of the creature, it can no longer make opportunity attacks against you until the start of your next turn. 

Level 15:

   Improved Venom: Your Venomous Fang ability now Poisons your target for 2d4 rounds, and they take 3d6 Poison damage on a failed Constitution save. Additionally, you can now apply this venom to a weapon as a Bonus Action. This poison lasts for up to an hour outside of your body until it is applied and consumes a use of your Venomous Fang ability. 

Level 18:

   Crippling Strikes: You can use precise strikes to take down prey much larger than you. When you make an attack against a creature, you may take a -5 penalty to the attack roll and attempt to cripple that creature, toppling it to the ground. YOu can choose to use this feature after you make the attack but before you know the result. If the attack hits, the creature is knocked prone. Additionally, the creature must spend a full action in order to stand back up. 

Information borrowed from Lycanthropy (House Rules): www.gmbinder.com/share/-N-7fAIa0tBcc3461W_U