Let's Read the 4e Monster Manual 3: Rot Grub
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Rog grubs have been around more or less since the beginning, though they used to be more of a trap than a monster in the past.
Rot grubs are voracious parasites with a life cycle so accelerated some scholars think they’re a divine curse sent by Torog to inflict suffering upon the world. They burrow into the bodies of larger creatures, eating their flesh and laying eggs inside. They don’t care if the body in question is alive or dead, either. A living being infested by a swarm of rot grubs will experience indescribable agony as they’re eaten alive, and this lasts until the grubs reach the heart and kill the host.
Rot grub infestations usually happen in underground dungeons, since they like the dark and humidity. Sapient creatures that like the same environments quickly learn to watch out for rot grubs, since an infestation can depopulate their homes in short order. Some, like orcs, just pull up stakes and move elsewhere when they find rot grubs. Others try to find uses for them. Kobolds stick them inside clay pots for use as special sling ammunition. Bugbears use them to enhance pit traps. Gnolls use them as instruments of torture.
These infestations tend to burn themselves out as the rot grubs eat through all available victims in the area and then die from starvation. Only a few hardy individuals remain, possibly in hibernation, waiting for the next unwary victim to approach their corpse host.
There are, however legends of entire oceans of these horrible little things undulating in the depths of the underdark. Supposedly, they sustain themselves by feeding on the flesh of a dead god.
Rot Grubs are Natural Beasts, and a swarm large enough to count as a monster is Medium in size. These swarms have Speed 5 and Darkvision, so they’re far from the stationary hazards of earlier editions. If a swarm notices you, it’s going to pursue.
Rot Grub Swarm
The basic model. After killing their first few victims, they retreat inside the bodies and lay up to ten thousand eggs. These hatch into new swarms after a month or so. This is the classic “trap” scenario from earlier editions.
This swarm is a Level 4 Brute with 63 HP. It has the common traits mentioned above, and the usual Swarm traits: half damage and no forced movement from single-target attacks, vulnerable 10 to area attacks, can occupy the same space as other creatures and squeeze into any space big enough for a single grub.
It’s also a Clumsy Attacker, which means it lacks a basic melee attack. It still has an attack ability, but it’s not considered “basic”. This means the swarm can’t charge or make opportunity attacks.
Its Swarm Aura has a size of 1 and deals 5 damage to any enemy that stars its turn inside. This damage increases by 2 for each extra swarm whose aura overlaps the target.
The swarm attacks with Infesting Bites, which deal no immediate damage but inflict ongoing 5 damage on a hit (save ends). This worsens to 10 after the first failed save, and to 15 after the second. An unlucky level-appropriate adventurer will find themselves dead from this about as fast as a PC from the olden days. Save-boosting powers and items are a wise investment here.
Rot Grub Zombies
Even grub-infested corpses are a little more active than they used to be! This is such a horrible death that it’s not uncommon for the victim to rise as a zombie soon afterwards. These zombies are on the faster side, and some speculate they’re controlled by the swarm inside them, since they are drawn to victims with open wounds that make it easy for the grubs to leap into their bodies.
The zombie is a Level 8 Skirmisher with 86 HP and Speed 6. Its basic attack is a slam, and it can also execute a flailing slam that allows it to shift 1 square before or after the attack. Their special attack is Rot Grub Hunger (recharge 5+), which targets a bloodied enemy and deals slam damage plus 2d6 necrotic damage. Hit or miss, it also exposes the target to rot grub infestation.
When the zombie hits 0 HP, Corpse Collapse kicks in and spawns a rot grub swarm in its space.
Rot Grub Infestation is a level 4 disease with an improve DC of 18 and a Maintain DC of 12. This makes it a slower version of the infestation implied by the basic swarm. Stage 1 eats a healing surge and makes the patient vulnerable 5 to necrotic damage. Stage 2 eats a second healing surge and inflicts a -4 penalty to all skill rolls. Stage 3 kills the victim and immediately turns them into a rot grub zombie.
If you dislike the way D&D 4 implements disease, you could replace the exposure effect and bonus necrotic damage of Rot Grub Hunger with the increasing ongoing damage from the basic swarm.
Awakened Rot Grub Swarm
Maybe this is a swarm that ate something very psionic, maybe it’s a duergar bioweapon run amok… whatever its origins, this is a rot grub swarm that became sapient. Its telepathic hive mind emits constant chatter with affirming phrases such as “Without us, within them, reap their flesh”. Sometimes bits and pieces of its victim’s minds also contribute to the noise.
The awakened swarm is mostly identical to the basic one, but it’s a Level 10 Brute with 123 HP. All the numbers go up, including aura damage and the ongoing damage inflicted by their attack. They also get a couple of extra psychic powers. Psychic Shock is a Close Burst 2 that deals no damage but immobilizes for a turn on a hit. And Telepathic Lure is a ranged attack (minor action, 1/round) that pulls a target 4 squares.
And of course this swarm is sapient, with Int 8, which is plenty to allow it to set up ambushes or even make conscious alliances with other beings it can’t eat.
Ewwww. Early D&D modules loved using rot grubs as a sort of lethal trap hidden inside corpses found in the dungeon, usually those of previous adventurers. It was not uncommon to also place some enticing loot on these corpses. They can still be used in this role here, and while they’re not as instantly lethal as their older versions, they’re still quite dangerous.