The corpse flower is a horrible ambulatory plant that scavenges the remains of the dead and occasionally belches one of them back out as a zombie. As such, particularly given its slow speed, it’s not a predator that might pursue a party of player characters, but rather a noxious nuisance that the PCs might be called to eliminate.
It has a weird ability contour: peaks in Constitution and Wisdom, with high Strength and Dexterity as well. It has no Wisdom-based offensive action, and while its Strength and Dex are equal, its Tentacle attack is clearly Strength-based. What we have here, I think, is a creature that’s mostly brute but that also has a touch of the skirmisher to it. It’s not fast, but it can climb, so that’s going to add a wrinkle to its behavior.
The corpse flower’s Intelligence is just barely within the range of sentience. Though able to grasp what’s going on around it, it’s still extremely instinct-driven and inflexible, locked into one mode of behavior. What about that high Wisdom, though? Normally it would suggest a creature that’s good at sizing up threats and reluctant to pick fights it can’t easily win. However, the corpse flower is utterly reliant on its blindsight to sense danger. It’s not able to pick up on clues such as a character’s confidence or the quality of their equipment to read them as presenting an above-average threat. Any reaction to the danger an opponent poses is going to have to occur after a demonstration of that danger.
The corpse flower has two main actions: Harvest the Dead and a triple-Tentacle Multiattack. Harvest the Dead is situational, usable only when there’s an “unsecured dead humanoid” within its 10-foot reach. What does “unsecured” mean? On the level of the obvious, we’re talking a corpse that isn’t roped or chained down. But the flavor text states that corpse flowers prowl “battlefields and graveyards.” Corpses on battlefields are often just strewn around, but in graveyards, they’re usually buried or entombed, and possibly enclosed in coffins as well. Are these bodies “secured”?
I think so, and furthermore, note that while the corpse flower can climb, it can’t burrow. Nor does it have any trait or action that suggests an ability to dig. Per the Monster Manual, monsters can take the actions available to all creatures—i.e., those listed in “Actions in Combat” in chapter 9 of the Player’s Handbook—so we could stretch that and argue that by taking the Use an Object action, a corpse flower could wrench off the lid of a coffin or yank open the door of a tomb, perhaps upon succeeding on a Strength check. I’m disinclined to do that, though.
To a corpse flower’s blindsight, a coffin looks like a wooden box; a tomb, like a solid block of stone; a grave, like any other patch of ground. How could it know there was something it wanted inside? It doesn’t have Keen Smell, nor any other specialized trait that would let it sense the presence of a corpse that wasn’t lying out in the open. A partially destroyed coffin, lying aboveground—that would be another matter. It might use a tentacle attack to smash it to flinders, then Harvest the contents on its next turn … but only if there weren’t easier pickings in the area, and only if it wasn’t under attack. The battlefield is therefore much preferred over the graveyard.
The corpse flower’s preferred action is Harvest the Dead, because taking this action makes it stronger, so whenever it’s given leeway to do so, its turn sequence is simple: move, snatch corpse. However, when anything moves within its reach, it reflexively lashes out with its tentacles. If only one creature comes close enough, they get whomped by all three tentacles. If three creatures approach, they get one tentacle each. If two approach, they get one each, and one or the other gets whomped once more, with enemies not incapacitated by the corpse flower’s Stench of Death being favored as targets. Otherwise, the selection is random.
The whomping doesn’t stop when a foe is beaten unconscious. An unconscious humanoid is of no use to a corpse flower. A dead one is useful. Therefore, without compunction, a corpse flower keeps attacking an enemy who’s reduced to 0 hp—focusing its attacks on this enemy, sensing that they’re near death. As soon as that last death save failure is chalked up, it Harvests their body on its next turn, even if this means forgoing an attack against another living enemy.
Like the shambling mound, the corpse flower doesn’t know what to make of a ranged attack that originates from beyond its blindsight radius, and it reacts instinctively to a hit from such an attack by moving away from the attacker. A ranged attack that originates from within its blindsight radius, however, draws its ire and causes it to move toward the attacker.
One trick the corpse flower employs instinctively whenever it’s under attack is to move toward the nearest tall thing—a tree, a tomb—and climb up it until it’s 10 feet off the ground. This movement places it out of reach of its opponents’ weapons, while they remain within reach of its tentacles. It can also snatch a corpse up off the ground if and when it creates one.
In addition to its action and its movement, the corpse flower has a choice of bonus action: either pop out a zombie or regenerate some hit points. Until it’s moderately wounded (reduced to 88 hp or fewer), the corpse flower doesn’t bother to regenerate; as long as it senses enemies, it uses that bonus action to chuck a zombie at them. After it’s moderately wounded, it digests a corpse and regenerates anytime it’s taken 11 damage or more since the end of its previous turn. If it’s taken less damage (or none), it produces a zombie.
(Yes, it pops out zombies even when it’s up in a tree or sitting on top of a tomb. Yes, the zombies take 1d6 falling damage as they plummet to the ground. Falling zombies are comedy gold.)
The only time the corpse flower doesn’t use a bonus action at all is when it’s uninjured or only lightly wounded and doesn’t sense another living creature within 120 feet. I mean, within reason—let’s say a Small or larger living creature. A bird flying over it isn’t going to activate zombie production.
Then again, this condition raises an interesting question. Suppose something bigger, like a coyote, passes through the corpse flower’s blindsight radius and does trigger the production of a zombie, but then the triggering creature simply leaves. That’s going to leave an unattended, purposeless zombie wandering around, isn’t it? Yep. So I’d conclude that any corpse flower encounter is always going to be foreshadowed by at least one seemingly random zombie, and once a group of PCs finally closes in on the thing, there ought to be a few already existing zombies to deal with as well.
When a corpse flower is seriously wounded (reduced to 50 hp or fewer), its self-preservation impulse kicks in to tell it that it’s not going to survive just by digesting its corpse collection. Lacking the Intelligence to Disengage, it Dashes away in whichever direction it senses the fewest enemies in.