Phase Spider

A phase spider on the party's side is unusual but not impossible. A drow noble has gifted one as a hunting companion, a Lolth-pact warlock keeps one as a familiar-in-all-but-name, or the party found a clutch of eggs and raised one (good luck). The spider does not love anyone. It tolerates the party because it has been shown that the party feeds it.

In a fight the spider's value is the bonus-action Ethereal Jaunt. Send it through a wall to scout the next room. Have it drop into the back rank of an enemy formation, take two Bites at a vulnerable caster (+5 to hit, 17 average per Bite, with the kidnap clause if either Bite drops the target), then Ethereal Jaunt out before the enemy melee can react. This is the assassin pattern, and the party should treat the spider as a one-shot deletion tool aimed at high-value enemy targets, not a brawler. AC 14 and 45 HP do not last in a stand-up fight.

Out of combat the spider is a scout no wall can stop. Ethereal Sight 60 ft. plus Ethereal Jaunt means the spider can phase through a closed door, look around the next room from the Ethereal Plane, and phase back to where the party is waiting. It does not speak, so signaling has to be worked out: one tap for clear, two taps for danger, that sort of thing. The player who runs the spider should be the one to invent the code.

The spider will eat anything the party leaves out. Plan accordingly, especially around captives.

A phase spider is an ambush predator that lives in two places at once. The whole encounter design rests on Ethereal Jaunt as a bonus action, which means the spider can spend an entire fight teleporting between the Material and Ethereal planes, attacking only on its turn and then vanishing before the party can answer. Run it as a horror beat first, a combat encounter second.

The opening turn is the spider stepping out of empty air next to the wizard, biting twice at +5 for 17 average damage (8 piercing plus 9 poison), and then bonus-action Ethereal Jaunting back out of reach. If a Bite reduces the target to 0 HP, the target becomes Stable, gains the Poisoned condition for 1 hour, and is also Paralyzed for that hour. That is the part the players will remember. A downed PC is not just down, they are kidnapped, and a phase spider will drag a paralyzed body away to web up for later. Make this happen at least once, on a PC the party can probably get back.

Positioning matters because Ethereal Sight only sees 60 ft. into the other plane. The spider does not know where the cleric is if the cleric is more than 60 ft. from where the spider crossed over. The party can exploit this once they figure it out, splitting up across rooms or running into the next chamber. Spider Climb and Web Walker matter if you have set the lair full of webs, which you should: webs are difficult terrain for the party and a navigation grid for the spider, and Web Walker means the spider knows where every PC who touches a web is standing.

Phase spiders flee when Bloodied. Ethereal Jaunt is a perfect retreat tool. Disappear into the Ethereal, move 60 ft. through a wall, come back somewhere quiet to heal. They will return when the party is mid rest, and the second encounter is the harder one because the players know exactly what is coming.

Have the spider take something the party will want back. A pouch, a holy symbol, a familiar. The pursuit is more interesting than the kill.