Cursed Mastiff

Cursed

At night, a quick burst of purple flames surround the dog. It shrieks in pain, but the flames quickly dissipate. What's left behind is the same dog, now black, with not one but two heads. It looks scared but also more menacing than before.
The effect will end in the same burst of purple flames as soon as the sun dips below the horizon, leaving behind its regular mastiff form.

Daytime

Keen Hearing and Smell. 
The mastiff has advantage on Wisdom (Perception) checks that rely on hearing or smell.
Skills Perception +3
Senses Passive Perception 13

Nighttime

Two-Headed. The dog has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.
Tough. The dog gains 5d8+11 Hit Points at night. (roll every night)
Skills Perception +5, Stealth +4
Senses Darkvision 120 ft., Passive Perception 15

Daytime

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) piercing damage. If the target is a creature, it must succeed on a DC 11 Strength saving throw or be knocked prone.

Nighttime

Multiattack. During nighttime, the dog makes two bite attacks.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage. If the target is a creature, it must succeed on a DC 12 Constitution saving throw against disease or become poisoned until the disease is cured. Every 24 hours that elapse, the creature must repeat the saving throw, reducing its hit point maximum by 5 (1d10) on a failure. This reduction lasts until the disease is cured. The creature dies if the disease reduces its hit point maximum to 0.