How many people know someone who is afraid of spiders ? I think just about all of us do. It is a common fear. Now lets take your fear. Make it bigger then your average dinning room game table and has the ability to seamlessly shift between this plane and the Ethereal! Many players dread facing phase spiders. And if they don’t you are not doing it right ! These lovelies should scare the pants off most parties when encountered. If used properly at the moment realization of what is about to come their way a game master should hear a satisfying and slightly audible groan of dread sweep over the room.

So enough about my personal feelings of joy at the terror these guys bring. Lets take a look at what exactly it is about the Phase spider that makes it so deadly. This is a spider form the Ethereal Plane that hunts in the Prime. So instead of hanging out in a web waiting for you to stumble into its trap. A Phase spider sits in its own plane of existence where it is 100% silent and invisible to the players. Waiting for the right moment to strike.  Add to the mix that they are intelligent so they can plan and asses the danger to themselves and their companions before they attack. Giving them the upper hand more often than not.  And they can shift from one plane to the other as a free action and back again as a move. Thus unless you are standing there. Waiting for a Phase spider to attack you. he will appear, strike and phase away. And allow his poison to do its work. As this is going on he will move on to the next party member and do the same. In both Pathfinder and D&D the poison of a Phase Spider is a terrifying thing.  A Phase Spiders poison can drain up to 16 points of CON ! And the save to resist the poison is CON based.  So with each failed save the chance of resisting the poison drastically lessens.
As combat goes The Phase Spider has a Decent Armor Class and the likelihood that the first round of combat will be Surprise round. The spider comes into most fights with the upper hand. Adding a High Initiative roll of +7 there is a fairly good chance the spiders will get at least two rounds of flat-footed attacks on a majority of the party.  Just enough time to dose up most of the group with poison and phase back to watch the players wither under the effects. moving into position to phase back in and strike targets that are more resistant to the poisons effects.
As intelligent creatures they can sense when they are over matched and thankfully will often leave well before they are threatened with the loss of life. That said this could lead to a series of ambushes by a particularly evil GM. I would not often suggest this as It would seem that unless there is nothing to eat in the region and the players are the only other food. There are many things that the Phase Spider could turn to for survival if the party proved to be too tough to handle in the first encounter.

A few encounter & tactic ideas:
1. A Phase Spider has set up home near a patch of Violet Fungus. Shifting in and out of Phase and renaming close to the fungus patch. As the party deals with the violet fungus the spider will shift out and Poison anyone that makes it to the edges of the Fungus patch then Phase back to look for its next opportunity.
2. A child has gone missing. The party has been hired or asked to seek her out. Hungry Phase Spiders in the woods have found her. In an effort to feed more than just one of the spiders they have scared her into the hallow of a tree. Appearing from time to time to torment her and make her scream. Thus bringing larger food to her aid.
3. As the party makes their way along a narrow path on the side of a cliff face. Phase Spiders Ambush the party. One from the front and one from the back. They will attack from above as well as they shift in and out of Phase climbing along the walls and on the path. The path is only wide enough to fight in single file.
4. One of the party members in the dungeon sees something glittering like gold or gems on a ledge just a few feet out of reach. They can climb up the broken wall to get at it but a Phase Spider has laid an ambush where it can strike the party member when he is the most vulnerable. The Spider will poison the fist victim and then move from the hole down to the party member in the back of the group. Hoping that they will think the attacking spider retreated deeper into the location of his first attack.
5. Goblins approach the party’s druid with a flag of truce begging the players to figure out what is taking their people in the night. Think of the CHILDREN they say !
6. A Phase spider has begun following a Will-O-Wisp in the swamps using the ability’s to draw off prey much like spiders today hunt at night near lights and feed on lone victims.