Aquatic Lernaean Hydra

The race descends from a single progenitor, here called the Drowned Matriarch: eighteen flat, oval serpent heads with eyes like black onyx beads, each on a neck roughly a hundred feet long and as thick as a tree trunk. The necks join a snake body nearly 10 feet thick and another hundred feet long. Her heads are ultramarine, and the necks and body are banded in that same blue and a greenish gray, a pattern that vanishes against choppy water.

Her spawn share the build but come in blue, sea green or black, marked with lighter bands or spots, usually gray, ochre or olive.

These hydras lurk in shallows and seize anything moving along the shore or across the surface. Each head is worth 1 Hit Die and 8 hit points. Every 8 points of damage kills a head, and unless fire reaches the stump, two heads sprout there in 1d4 rounds, up to a maximum of eighteen. Sealing a stump takes a flame at least the size of a torch and a called shot (-1 initiative, -4 to hit). Directional fire such as burning hands seals a stump with no roll needed; a flame tongue sword also works, but the wielder must make a separate called shot.

The beast knows fire is its enemy and stays in the water, holding wounded necks under until the new heads emerge. Unless it comes ashore or the party can produce fire underwater, a stump must be burned in the same round the head falls.

Area spells kill one head per 8 points; any remainder is split evenly among the heads still standing, and the creature makes just one saving throw. Heads lost to fire never double. A successful disintegrate, slay living or finger of death kills it outright and stop all regrowth. Blows to the body accomplish nothing, and nonmagical cold does not affect it.

Each head gets one attack, and THAC0, damage and XP scale with the starting head count: 5 heads, THAC0 15, 1d6, 1,400 XP; 6 heads, THAC0 15, 1d6, 2,000 XP; 7 or 8 heads, THAC0 13, 1d8, 3,000 or 4,000 XP; 9 or 10 heads, THAC0 11, 1d8, 5,000 or 6,000 XP; 11 or 12 heads, THAC0 9, 1d10, 7,000 or 8,000 XP. After a fight its head count drifts back to normal at one per day, extra heads dropping off and burned ones regrowing.

Most of the day it floats at the surface waiting for a meal, and it sleeps on the lake bed. It is savagely territorial, chasing off any other big predator. Every one of these hydras is descended from the Matriarch, so for now they live only in waters linked to her lake, though they can travel overland and could turn up nearly anywhere.

They eat only fresh kills, can fast for months, and gorge when they get the chance. Air and water are equally breathable to them. Each mates a single time and then bears 6-8 live young every year, living about 300 years. The Matriarch herself can grow as many as twenty-four heads.