Mandrake

Mandrakes are humanoid plant creatures that can instantly kill anyone that hears their mind-rending scream. Once they reach adulthood, mandrakes stalk their forest and swamp lairs in pursuit of anything that moves, fertilizing the ground with their corpses. Even the most formidable beings can fall prey to a mandrake’s death wail, and the wisest course of action is to avoid the telltale sound at all costs.
       Bitter Hunters. Juvenile mandrakes are fragile parasitic creatures, reliant on a host to provide the nutrients they need to survive. These nutrients are transferred through a specialized umbilical vine, which eventually kills the host as it siphons away its life. If the suckling mandrake and its tether are kept intact through the process, it undergoes a transformation. The mandrake constructs a cocoon for itself near the corpse and, one week later, emerges as an adult mandrake.
       The host that nurtured the mandrake transfers the sensation of its agonized death through the tether, twisting the mandrake’s form to be corrupted and torturous. The mandrake’s nonstop infantile wailing matures to become a note of pure hatred, which pierces the psyche of those who hear it with enough psychic malice to instantly stop the heart. If its scream doesn’t kill you, the adult mandrake uses its newfound strength to maul enemies to death. These vile plantoids hunt out of equal parts hunger and enmity for the cruel world that birthed them.

Variant: Mandrake Seedbearer
Some mandrakes can disperse their seeds with a vine appendage that ends in a barbed seed pod. If you’d like to increase the difficulty of an encounter with a mandrake (+1 CR), consider making it a seedbearer by adding the following action to its statblock:

Seed Whip (3/Day). Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 6 (1d6+3) piercing damage and the target is implanted with a seed.
       A seed implanted into a creature is undetectable unless discovered with a successful DC 20 Wisdom (Medicine) check (no action required), and can be removed with an action. If a seed is removed it dies. The mandrake knows the location of each implanted seed at all times, even on another plane of existence. As a bonus action, the mandrake can cause an implanted seed to grow suddenly, dealing 2d6 necrotic damage to the host. A suckling mandrake then bursts forth, tethered to the host, and lands in an unoccupied adjacent space. A seed automatically undergoes this process if left implanted for 1 hour. The suckling mandrake takes its turn immediately after the host and is friendly to the mandrake.