|Summoning a Nightstalker|
A significant amount of my mental landscape when it comes to fantasy was laid by the old Microprose game Master of Magic (1994). Back in elementary school before I had any actual D&D rulebooks, I made up monsters according to their stats in Master of Magic; and though I may have ultimately switched rather completely to 3E in highschool, and now to B/X in 2012 onward, still Master of Magic stands as an ur-source, like The Lord of the Rings, or the Timelife Books: The Enchanted World series, or The Flight of Dragons cartoon.
The Nightstalker is a summoned creature from Master of Magic, which is in hindsight clearly a rip-off of the invisible stalker in D&D; and yet, the Nightstalker is undead, only able to be conjured with a wizard with Death spellbooks; moreover, the Nightstalker has a formidable death gaze attack, which the invisible stalker clearly lacks.
Here's the Nightstalker's abilities per Master of Magic (drawn from a PDF download of the manual):
Nightstalker -- Magic (Death) -- Upkeep (1 Mana) -- Melee (7) -- Defense (3) -- Resistance (8) -- Hits (10) -- Movement (2) -- Special (Death Gaze -2 save; Invisible)
From which I will draw this (soft) D&D version:
AC 0 (4 if one can see invisible)
damage 2-12 + death gaze
death gaze -- all characters in melee with the Nightstalker save against death or die (+2 on save, -2 "to hit" if characters look sideways; +4 to save, -4 "to hit" if characters fight with eyes closed/face averted/etc.)
save as a Fighter 6
immune to charm, sleep, cold
turned as an undead of HD+2 (i.e. 7 for this write-up)
Such a monster is, of course, terrible to use against a party that has no way to detect invisible ... and nevertheless, I wrote a small dungeon that included something along these lines as a possible wandering monster within (though that version was for 5E, rather than B/X D&D)
Naturally, I rolled its appearance a couple times, and whenever it showed up, the party was in a high dudgeon of terror given the fact that it's naturally invisible and can strike when it pleases--and with a death gaze to boot! Two lower level compatriots died under the horror of its death-gaze; and ultimately, the party looted what they wanted without ever killing the invisible monster stalking the dungeon below ...
The stats don't necessitate any particular HD, AC, or anything, I'm just spitballing according to what I know of the Master of Magic wargame system, and still downplaying the monster a bit. The Nightstalker could easily be scaled up or down depending on what you want to do with it--maybe it's semi-visible for low level parties (note that it doesn't have immunity to normal weapons, and may be struck by any old sword!); maybe it has HD 10 for a high level group that may or may not have see invisible.