Breath Weapon.  It can forsake its bite attack to let lose a 10 ft cone of diseased acid that deals (2D6) acid damage and can inflict a sickening disease that causes (1d2) con damage.  Each creature in this cone  must make a DC 14 Dex save .  On a failed save  full acid and con damage is taken or half as much damage  and no Con damage.  Con lost this way can be restored by a long rest or a lesser restoration spell.  Breath weapon recharges (5-6)

Keen Smell. The rat has advantage on Wisdom (Perception) checks that rely on smell.

Pack Tactics. The rat has advantage on an attack roll against a creature if at least one of the rat's allies is within 5 feet of the creature and the ally isn't incapacitated.

Multiattack. The undead rat can make two attacks: one with its bite and one with its claws.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit:  8 (1D6 + 4) piercing damage plus (1d8) poison damage.

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (1D6 + 4) slashing damage.