Multiattack. The death dog makes two Bite attacks.

Bite. Melee Attack Roll: +4, reach 5 ft. Hit: 4 (1d4 + 2) Piercing damage. If the target is a creature, it is subjected to the following effect. Constitution Saving Throw: DC 12. First Failure: The target has the Poisoned condition. While Poisoned, the target's Hit Point maximum doesn't return to normal when finishing a Long Rest, and it repeats the save every 24 hours that elapse, ending the effect on itself on a success. Subsequent Failures: The Poisoned target's Hit Point maximum decreases by 5 (1d10).