Alchemical hydra guide
The Alchemical hydra is a level 426 boss, with 1100 health. To access it you must complete the quest "Creepers in the Crevice" which requires 95 slayer to complete.
Creepers in the crevice quest completed
The most notable rewards from Alchemical hydra are
Ikkle hydra pet
and the hydra's eye, fang, and heart to make the Brimstone ring
The Alchemical hydra has 4 different forms which each have its own special attacks. The first form is green and for its special attack it will spit venom pools at you, this is pretty easy to dodge by moving a few tiles away but will quickly reduce your hp if you stand in it too long. The second form is blue and its special attack will have lightning storms in the room that deal damage when too close to you. The third form is red and will cover a large area with fire as its special attack, then shoot a fireball at you which will stick and deal continuous damage unless you're moving. The fourth and final form will turn black and will also spit venom pools at you.
Each hydra form except the last one can be slightly weakened by luring it on to one of the colored vents on the floor in the room, the south east red vent is for the first form, the north east green vent is for the second form and the north west blue vent is for the third form.