RSA/RMA: Invisible Stalker

It's a spell! No, it's a monster! No, it's a spell and a monster! It's the Invisible Stalker! It's a Random Spell + Monster Assessment!

This one has been around for nearly every edition of D&D that I know of, yet it seldom comes up in play. The BX version of the creature only appears if summoned via the 6th level MU spell of the same name, which may be unique for a creature listed in the monsters section; unless you include elementals, which can be summoned in three different ways (staff, device, or spell).

Invisible Stalker (from Cook)

AC: 3
HD: 8*
Move: 120' (40')
Att: 1
Dmg: 4d4
No. App: 1
Save: F8
Morale: 12
Treasure: Nil
AL: N

Since it's summoned via a 6th level spell, it makes sense that this is a fairly tough creature: good AC, high HD, respectable damage, and infallible morale score are enough to make a bad time for anyone the stalker is sent after or who gets in its way.

The stalker is summoned via spell to perform some task for the magic-user. It could be to kill someone, but it might be an entirely different objective. Unlike automatons like zombies or golems, the stalker is 'very intelligent.' So much so that it may try to subvert the spirit of the command while obeying the letter of it because it resents being summoned (much like an Efreet).

Other factors that make the stalker so dangerous is its innate invisibility. While BX does not go into detail, other editions describe the ability as persisting even if the creature attacks and not being affected by dispel magic. Further, it surprises on a 1-5 (thanks to the invisibility) and is described as a "faultless tracker" (similar to an Amber Golem).

There is no duration to the spell. The Invisible Stalker remains until its task is complete, it's slain, or the 5th level clerical spell dispel evil banishes it. A scenario that requires the presence of fairly powerful characters: a 7th level cleric for banishing and/or an 11th level MU for summoning. This implies that the creature really shouldn't be too common before PCs have a few levels under their belts. ISs do not appear randomly; in the wilderness or in dungeons. They should be placed deliberately if they show up at all (unless summoned by a PC).

As a DM, I can imagine scenarios where an NPC wizard summons an IS and sends it on a mission that conflicts in some way with the PCs' goals. It isn't sent specifically to kill the party, but it will eliminate obstacles as necessary. Remember that stalkers are intelligent, they will solve problems creatively.

Likewise, I expect a scroll or other non-permanent item that allows summoning an Invisible Stalker falling into the party's hands could cause a good deal of plot-fodder. If they aren't careful, they may find that they don't quite get the results they hoped for.