Shambling Dead

Undead Fortitude. If damage reduces the shambler to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the shambler drops to 1 hit point instead.

Horde Formation. Over time, shamblers will naturally group together, harkening to the need to be social while they were alive, growing from a pair to a handful to ten, twenty, and larger. Hordes may collectively appear intelligent as they hunt.

Time Reclaims All. Within 5 years, the first Shamblers will have rotted enough to no longer be a moving threat.  Another two years, and they will be no more than a desiccated corpse.

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.

Horrific Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: (1d6 + 4) piercing, disease infection happens when attack roll is 4 higher than target AC. This does require a successful grapple, with advantage when the target is flanked by other shamblers. The infection inflicts 6 disease damage per day for 3 days, long rest is impossible, only a Greater Restoration spell can remove the disease. If the target dies during the 3 day period, they will rise as a Shambler and attack any living creature in their vicinity. On the third day, if untreated, the victim is human, and they had to make death saves over the recovery period, there is a chance to transform their race to an Ender (players can make this choice, NPCs are 50% chance or at discretion of the GM).  Those who survive and do not become Enders gain a DC 15 Constitution Save against new infections, reducing the DC by 5 for every time they survive each subsequent infection.