In Valley of the Brain Collectors there is a random encounter table. I combined this encounter with the Terrantula encounter – they turned up to feast on the carcass of the mega-spider. My players, in good faith, decided to kill them too. The world didn’t need anymore house sized spiders OR horse-sized wasps.
Large beast, unaligned
Armour Class 16 (natural armor)
Hit Points 93 (11d10 + 33)
Speed 30ft., fly 60 ft.
STR 19 (+4) DEX 10 (+0) CON 16 (+3)
INT 5 (-3) WIS 11 (+0) CHA 5 (-3)
Skills Perception +4, Stealth +4
Senses darkvision 60ft., passive Perception 14
Challenge 5 (1,800 XP)
Implant. A spider eater grows its eggs inside of a living host. Implanting an egg in a host is a full-round action that provokes attacks of opportunity, and the target must be helpless but alive. Once an egg is implanted, it exudes paralytic enzymes that not only keep the victim in state of perpetual paralysis, but also keep it nourished and alive in its comatose but fully aware state. This condition lasts until the egg hatches in 1d6 weeks, at which point the young spider eater consumes most of its host, killing it. An egg can be surgically removed with a DC 25 Wisdom [Medicine] check (this check deals 2d6 points of damage to the host regardless of success), at which point the host recovers from the paralysis in 1d6 rounds. Any magical effect that removes paralysis or disease (such as restoration) also destroys the egg, but mere immunity to paralysis or disease does not offer protection.
Multiattack. The spider eater makes four attacks: two pincer attacks, one bite and one sting attack.
Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature.
Hit: 8 (1d6 + 5) piercing damage, and the target must succeed on a DC 13 Constitution saving throw or be poisoned for 24 hours. Until this poison ends, the target has disadvantage on saving throws to resist the spider eater’s sting.
Pincer. Melee Weapon Attack: +6 to hit, reach 10 ft., one target.
Hit: 11 (2d6 + 4) bludgeoning damage.
Sting. Melee Weapon Attack: +6 to hit, reach 10 ft., one creature.
Hit: 5 (1d6 + 2) piercing damage, and the target must make a DC 13 Constitution saving throw or be poisoned for 1 minute. Until this poison ends, the target is paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Creatures with the arachnid tag have disadvantage on this saving throw.
This strange beast resembles a wasp the size of a horse, but with the head of a spider and two long appendages ending in pincers.