Can You Catch Him with a Fishhook?

I've bashed together some house rules for fighting giant insects in my campaign. This is an early draft of what I've imagined, so it needs some editing before I call it done. It also lacks testing, and may be terrible. Caveat emptor.

D&D deals with fighting similarly sized combatants well, but it doesn’t address dispatching truly vast foes in any codified way. By vast foes I mean enemies a PC could be expected to scale in the midst of combat. Fighting huge monsters should feel different than normal combat, it should be a struggle as much against scale and physics as it is force of arms. This supplement is intended to add some extra crunch to taking down colossal foes. It takes its main inspiration from the kind of set-piece conflicts found in Moby Dick, Shadow of the Colossus and Monster Hunter.  These rules focus on giant insects since they’re the most common type of giant monster in my home campaign, but I’ve tried to write this with conversion to other types in mind. Giants are totally pissed when one tries to climb them with pickaxes.

Tools of the Trade

There are several pieces of equipment that are more or less critical to fighting giant monsters. Hunters need plenty of rope to scale monsters, bind them, and to hold on to if the beast tries to throw off its tormentors. Grapnels are needed in tandem with rope, to catch on the carapace of the thing and allow safer climbing. Harpoons can be used in a similar manner to grapnels, with the added bonus of also dealing damage like a javelin. Once a hunter has scaled the beast iron spikes or stakes secure the line against the beast snapping free. Secured lines also make falling off much more difficult. Several types of pick are also useful, the small one-handed adze, larger two-handed mining pick, and long-handled alpenstock. Each is used as a climbing aid and to prise sections of chitin open to expose weak points on the body. Once exposed these section can be spiked open, allowing for sustained attacks.

Too Many Legs

When converting a monster to this system divide the total HP of the monster into two parts:
    
    30% Each limb (ex. A 9 HD Giant Carnivorous Beetle has 44 total HP, its limbs have 13 HP each)
    70% Body (ex. A 12 HD Hamlet-Eating Caterpillar has 67 total HP, its body has 47 HP)

When a limb is reduced to 0 HP it is considered detached. Insects reduced to half their original limbs are -2 to hit and AC, and move at half speed. Monsters with no limbs are unable to attack or move. When the body is reduced to 0 HP the creature is dead.

Girt in Iron Scales

When attacking from the ground all PC hits will strike the limbs, unless attacking from the rear. Due to the hardened carapace of giant insects all hits on the body and limbs not made with piercing weapons (picks, adzes, crossbow bolts, short range arrows, etc) or two-handed weapons deal half damage. If a PC is able to attack the head or a weak point, either by scaling the beast or through a lucky ranged shot (the head, being generally smaller and more active, is -4 to hit with a ranged weapon) the hit deals normal damage.

The Hunt Begins

The actions listed below are taken in place of normal movement and attack actions. DCs are listed, to be used at the DM’s discretion.

Movement Actions

DC10 Grapneling a line onto an unaware monster

DC 15 Grapneling a line onto an aware monster

DC 15 Scaling a monster with a rope

DC 20 Scaling a monster without a rope

Attack Actions

AC Drive an iron spike or stake into the monster, for securing a line or spiking a weak point

AC Expose a weak point, if wielding a piercing weapon.

We Need More Rope!

If the destruction of the monster isn’t the aim, it can be rendered immobile if tied down by a number of ropes equal to its HD. These ropes must be spiked down, or held by HD/3 characters. A giant monster can attempt to dislodge any characters scaling it, and also tear free binding ropes, by making a flail attack. This replaces one of its normal attacks. The DM makes a normal to-hit roll, comparing the total to the AC of all PCs climbing on the creature. Characters holding on to secured lines or embedded weapons (harpoons, spear, etc) add 2 to their AC for this attack. Staked ropes count as unarmored targets (AC 10). Ropes secured by characters count their AC as 10 + 1 per character. Any characters ‘hit’ by this attack are flung off the beast and take 1d6 falling damage (or more, depending on the height of the creature). Any ropes hit tear loose, and 1 in 6 will snap and become useless.