BX Monsters A to Z: Invisible Stalker

 The invisible stalker... a monster I have never, ever used in any game I've run or played in. Let's see if that was reasonable, or a missed opportunity.

An invisible stalker pursuing its quarry through ancient ruins.

Invisible stalkers appear only due to being summoned by a magic-user with a powerful spell. The stalker is a formidable creature, having an AC equal to plate armor and 8 Hit Dice, just one fewer than an elephant. It moves at the same rate as an unencumbered human, 120' (40'). Its single attack deals an impressive 4-16 (4d4) points of damage. And its morale is 12 -- it doesn't give up, ever, until it's slain or dispelled back to its home plane. (In other editions, this is specified as the Elemental Plane of Air, but in B/X, it could be any place the DM wishes.) Only a single one is encountered a time.

The signature characteristic of invisible stalkers, is, of course, invisibility, but this literally goes completely unremarked in the monster description, save a note that they surprise on a roll of 1-5 on 1d6. (This implies that there are other telltale signs of a stalker's presence which might be noticed, but again, no mention of what those might be -- noise? scent? tactile disturbances?) Presumably, the stalker's invisibility, being an innate quality, isn't dispelled on attacking, but this isn't stated explicitly either. I searched in vain for generic advice on invisibility in combat, and failing that, for guidelines on blindness, but found none in B/X. The Mentzer rule sets, if one looks hard enough, say that invisible creatures generally cannot be attacked, except by creatures under the direct influence of a detect invisible spell or guided by someone under such a spell, with the latter case imposing a -4 penalty to attacked and a +4 bonus to the invisible creature attacking them. (By comparison, the pixie, detailed in the Basic Rules, is explicitly noted to be exempt from being attacked in the first round, but can be attacked at a -2 penalty thereafter. Mentzer boosts this to -4.) In any case, figuring out how the stalker's invisibility applies in combat by-the-book is an onerous task, and still requires a lot of DM judgment.

Stalkers are noted to be prone to creative interpretations of their orders if given particularly open-ended or onerous tasks, reminiscent of djinn, efreet, and similar extraplanar servants. Aside from that caveat, they are otherwise undeterrable from their mission, except by "killing" their earthly forms or by means of a dispel evil spell. 

Invisible stalkers are stated to be "faultless trackers" which seems to imply that they'll always be able to track down their quarry, regardless of any other factors. No other information is provided, which does raise some questions. Do they track with conventional methods of Prime Plane life, e.g. detecting and interpreting impressions in the ground and damage to plant life, or by scents left behind, or by some unspecified extrasensory or magical means? Does it have limits to its range, or does the stalker someone sense its prey across infinite distances? Does flight or teleportation throw them off? What about plane-hopping?

The description also states that the most common use of invisible stalkers is to track and slay enemies. While certainly a viable use, it's hard to believe that the highly intelligent wizards who are capable of summoning the creatures in the first place would so readily discount other valid uses. Retrieving items or information from difficult-to-access places is a big one. Why spend gold to hire a human thief when you can simply cast a spell to draft a stalker into your service? A being that can slip into a restricted place unnoticed and lurk there indefinitely until an opportunity to rifle through the contents presents itself would make a hell of a spy or repo man. The only thing it can't do is pick locks, but given its hefty damage potential, it seems as if it would have the strength to pry open most doors and containers. So, hiring a thief might still be the better bet if you want to do a job cleanly, but if you don't care about leaving no traces, an invisible stalker is a perfectly good alternative, and honestly, less likely to doublecross the caster than a thief would be. 

What is most glaringly "wrong" with the invisible stalker's entry in the B/X bestiary is the lack of vital information on its invisibility, as well as any vulnerabilities or immunities it seems plausible it might possess as an extraplanar creature. At minimum, it feels as if it ought to be immune to poisons, mind-affecting magic, and hold spells. Otherwise, it's a pretty cool creature for an enemy wizard to inflict upon a party, or for a party to encounter in service to someone else. How does a stalker act when, unbeknownst to the party, they're pursuing the same objective? And of course, it's also there if any player-character wizard should learn and use the sixth-level invisible stalker spell.