Shapechanger.

The wererat can use its action to polymorph into a rat-humanoid hybrid or into a giant rat, or back into its true form, which is humanoid. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Gnome Cunning.

The svirfneblin wererat has advantage on all Intelligence, Wisdom, and Charisma saving throws against magic.

Innate Spellcasting.

The svirfneblin wererat's innate spellcasting ability is Intelligence (spell save DC 10). The svirfneblin wererat can innately cast the following spells, requiring no material components:

At will: nondetection (self only)
1/day each: blindness/deafness, blur, disguise self

Keen Smell.

The wererat has advantage on Wisdom (Perception) checks that rely on smell.

Stone Camouflage.

The svirfneblin wererat has advantage on Dexterity (Stealth) checks to hide in rocky terrain.

Actions

Multiattack (Humanoid or Hybrid Form Only).

The wererat makes two attacks, only one of which can be a bite.

Bite (Rat or Hybrid Form Only).

Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage. If the target is a humanoid, it must succeed on a DC 11 Constitution saving throw or be cursed with wererat lycanthropy.

Shortsword (Humanoid or Hybrid Form Only).

Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage.

Hand Crossbow (Humanoid or Hybrid Form Only).

Ranged Weapon Attack: +4 to hit, range 30/120 ft., one target. Hit: 5 (1d6 + 2) piercing damage.

Svirfneblin Wererat