Astral Searchers are one of the real gems from the much-maligned 1e Fiend Folio. They are really quite terrifying if you think about it: disembodied spirits, roaming around the spirit world, trying to break into the material world, to find a body to possess.
While returning from astral travel, there is a chance that a bus-load of these things (4-24 of them!!!) will latch on to you, following you back into the material world. The FF postulates a 4% chance, but I'd say roll the good ole d20, them appearing on a 20 (a 5% chance).
Successfully following you into the material world, they immediately attack, trying to destroy your soul and take over your body! Yikes....
Clarifying the Origins
The FF speculates that Astral Searchers originate in traumatic events and spell-use in the astral plane. This is only the shadow of the real truth...
The true origins of Astral Searchers are the people who had their SILVER CORDS SEVERED while astrally projecting. Their spirits are then lost in the astral mists, and stuck there, unable to return to their true bodies. They will desperately latch onto anyone they can find, hoping to catch a ride back to the material world to reattain a physical form.
Clarifying the Mechanics
Essentially, what happens is, the Astral Searcher grabs onto the victim's body while it is on the astral plane, and "rides along" as it shifts into the material plane.
What they are talking about here is the Ethereal-to-Material transition. This transition happens when a physical body shifts phases from an astral state to a material state.
Purely "spirit-only" travel, projecting your astral body but not your physical body, would not involve any danger from these things, since, in that case, there is no astral-to-material transition.
Clarifying the Combat Mechanics
I must admit, I am a bit unsatisfied with the description of the combat mechanics. The problem is, they way they describe it, there is really no parallel for this anywhere else in the D&D rulebook.
Where exactly is the rule for "fighting an attempted spirit possession"??? According to the FF, here is how it works:
Your spirit has AC 5, but the same amount of hit points as your physical body. If it knocks you down to zero hit points, your spirit is considered crushed, destroyed forever, and it takes over your body....
For one, spirit possession does NOT involve the annihilation of the host soul. The invader "over-shadows" the host spirit, but if the invading spirit is exorcised, the host soul reasserts itself.
For two, isn't there some better mechanic for fighting spirit possession???
Spirit Possession mechanic of the Magic Jar
We already have a really good mechanic given in the 5th level magic user spell Magic Jar. This is precisely a spirit-invasion, hostile-take-over attempt of some else's body by an invading spirit.
In this spell, resistance to take-over is based on a savings throw, which is modified by comparing the INT+WIS scores of the two combatants.
The time of possession is also limited based on the relative INT+WIS scores. The victim gets a save every round, turn, day, or week depending on the comparable scores.
Also, the magician spell Mind Blank protects you from possession or soul trapping... Presumably the psionic defense of the same name would provide that protection, too...
This Magic Jar mechanic makes a lot more sense than the one described in the FF, so that is what I would follow.
The Astral Searcher could still attack physically with its 2hd form and 1-6 damage strike, but if it is going to attempt a spirit takeover, let's be consistent, shall we?
INT+WIS modifier for Savings Throw versus spirit possession:
Victim at +9 or more --> +4 on save, save each round
victim at +6, +7, +8 --> +3 on save, save each round
victim at +3, +4, +5) --> +2 on save, save each round
victim at even, +1, +2 --> + 1 on save, save each round
attacker at +1,2,3,or 4 --> no adjustment on save, save each round
attacker at +5,6,7,8 --> -1 on save, save each turn
attacker at +9, 10,11,12 --> -2 on save, save each day
attacker at +13 or more --> -3 on save, save each week