The phase spiderling swarm is based on the phase spider in the core book. They are baby phase spiders, fresh from the cocoon.
Phase Spiderling Swarm
Normal 6th level Spoiler Beast (swarm)
Hundreds of little mandibles +10 vs. PD (1 attack)—20 damage
Natural even hit: The spider swarm can make a rummage and filch attack against the target as a free action, even if the target isn’t staggered.
Rummage and filch +10 vs. MD (one staggered creature)—the spiders steal a random small magic item from the target, such as a rune or a potion.
Stolen items: If the spider swarm succeeds in stealing an item, they will break potion flasks (usually by teleporting up and dropping them) and drink the contents. For runes and other items, some spiderlings will break off from the swarm and try to disappear through cracks where they can't be chased.
Break potion: Move action. The spiders regain 10 hit points and gain +2 to the attack roll and 2d6 to damage with their next attack. (Customize the effect depending on the potion)
Swarm: Resist attacks against AC 16+
Short teleport: As a move action, the spider swarm can teleport anywhere it can see nearby.
AC 21 PD 22 MD 17 HP 60
If the party follows a phase spider into its lair to recapture their items, they will find 2d8 silk cocoons. If a PC decides to cut open a cocoon, roll 1d6:
(1) Skeletal remains and goo
(2) Rotten air and fungus spores (save or take 20 poison damage)
(3) a drained magic item
(4) a working magic item
(5,6) a swarm of phase spiderlings
If the PCs stage a fight in the lair, feel free to have missed attacks pop open cocoons to add more spiderlings.
The younglings are not quite as nasty as the mother, as they are one level lower and don't get double damage and hp for size. They also haven't developed full phasing yet. True magic items are too big for them to steal (except for rings and similar), so they will usually try to run off with small items like potions. They are still annoying enough when they leave the party without backup healing potions. As a swarm, it is not very effective to hack away at them with swords and other attacks against AC. Spells work much better.