Phase spiders are surprisingly similar to the previously-covered ethereal marauders. They're magical beasts that can move to the ethereal plane as a free action, then back as a move action, lack limbs, and have a bite attack.
Of course, there's differences as well. The spider has 5 rather than 2 RHD, and its ability scores are significantly better. It also has constitution-damaging poison riding on its bite, and a climb speed made mostly useless by its ability to go ethereal.
I'm thinking either +1 or +0 LA is best here. That way, the marauder is still a competitive option if all one cares about is getting etherealness as cheaply as possible, while the spider provides a more balanced base creature that doesn't lose as many HD.
+1 LA for now because etherealness is awesome at level 6, though I invite community feedback.
Phasms are next.