Source: Fortress of the Stone Giants, pg(s). 79
Deathwebs are made of the dead exoskeletons of truly massive spiders, and are normally at least as large as houses and sometimes even larger. They appear much like the spiders they were in life except now their huge exoskeletons are cracked and leaking swarms of poisonous spiders. They move faster than their unnatural, dead bodies should allow.
Habitats & Ecology
Deathwebs are undead, and as such have no real place in the natural world. Deathwebs are usually found underground, in mountain caves or beneath the canopy of ancient forests often these areas boasted old magic. While often created by powerful necromancers, worryingly deathwebs can rise spontaneously usually in areas crawling with insectile vermin or in areas that have played host to dark spider cults.