Feralgeist 5E Tactics

These undead are fun, possessing not *players* but dead bodies, making them a potential bane for "murderhobo" style groups that leave a trail of slain enemies behind them. The artwork is great, implying the entity has possessed the corpse of a winter troll from the look of it, which is interesting as this implies some necrotic damage was dealt to prevent the trolls significant regeneration ability - was the troll killed by the group that must now face the reanimated corpse in karmic retribution?

These *incorporeal* undead are quite different from the majority of Cryxian thrall based, Orgoth origin or Iosan descended corporeal undead that seem to dominate the Monsternomicon, 

Motivation wise they are not complex:

A feralgeist’s insubstantial form hungers for a body of flesh. It descends into the carcasses of freshly slain beasts and brings to them a semblance of reanimated life. Lashing out with the claws and fangs of the corpses it inhabits, the feralgeist poses a deadly threat to those unfortunate enough to encounter it...

Pretty standard for a chaotic evil ghost entity - as Keith proposes, undead are driven by compulsion as an overriding factor and for this creature, that seems pretty easy to work out and they seem more fanatical than most - after all, they mainly use other creature's corpses to attack. 
Let's have a look at the stat block analysis then.

So starting with the mental ability contour, because most of the time a feralgeist will be attacking with the style determined by the former ability contour of the corpse it is reanimating, the striking thing is the very high CHA score. 

Given the creature speaks no languages, has *no* social skill proficiencies, no CHA based spellcasting, and is not noted to speak or converse otherwise, this seems odd as INT is only break even average and WIS only slightly higher. The only thing I can think of is the utility for CHA save based effects which are only a limited number in core 5E, and given the Iron Kingdoms reduced role of planar magic, perhaps only bane is of significant relevance to a party of the relevant experience levels for the CR.  Magic circle doesn't actually require a CH save unless the creature tries to teleport into the circle which a feralgeist can't do, banishment doesn't work as usual in the setting, and while dispel evil and good forces the undead form the possessed corpse, it is a 5th level spell likely to be beyond most 2nd or lower tier parties. To be fair I haven't looked through all the new IK:R and B+B spells but there aren't any that spring to mind immediately. I don't think this is an error however, as CHA is linked to a creature's innate sense of self which lore wise appears to be very strong given its presentation. 

It's a bit hard to compare to an earlier edition version of the creature as the original 3.5E Monsternomicon only has the similar "wyldgeist" and it's not always easy to extrapolate between editions. 

Taking its flight (hover), Incorporeal Movement, darkvision, and Death Sense into account and noting the omission of proficiency in Stealth, the picture emerges of a spirit that hunts at night but isn't particularly discreet about its intentions, raging through the twilight or dark forests as an evil green mist. 
Physically, although it has only moderate hit points and lowish AC, the number of damage resistances (including all non-magical common physical attacks), damage immunity to three less common types (cold, necrotic, poison), and multiple condition immunities compensates considerably - it's much more likely to be driven off and survive (say when it reaches 10% or 6 or fewer hit points) than it is to be destroyed, making for a potentially recurring (but relatively unintelligent) recurring foe.   

As the feralgeist does not have a standard attack option, the physical ability contour is somewhat meaningless although the high DEX and above-average CON would suggest a shock attacker or maybe a skirmisher that doesn't really fit at all - if not animating a corpse it actually can't attack, so it's only options are to Disengage from any melee and use its faster movement and flight to either escape out of reach (moving away upwards, through a barrier, and/or into obscurement on subsequent turns) or less commonly move to the next corpse available if its Spirit Bind has already recharged.

Spirit Bind (Recharge 6) is the key to this creature and its tactics when considered in conjunction with its mental characteristics - as it targets corpses, *not* opponents, I'd consider that its average WIS and INT suggest it picks the largest, strongest looking corpse but isn't smart enough to pick one with specific traits or abilities that it can use.

If it is forced from its chosen reanimated corpse (most likely from being damaged rather than dispelled given that dispel good and evil is a 5th level spell), the feralgeist will avoid combat and being damaged as much as possible until its ability to reanimate a fresh corpse.     

Example: Winter Troll Reanimated Corpse

This is the possessed corpse shown in the 5E Monsternomicon artwork, a winter troll (which I haven't analysed formally yet - I plan to deal with trolls as a group in a later post), so let's see how this works as an example using the rules for Spirit Bind:

A feralgeist possessing this corpse gains all the physical abilities of the troll (apart from hit points), but has a relatively higher INT, lower WIS, and lacks proficiency in *both* Perception and Survival suggesting somewhat of a change in approach. It's the significantly lower hit points that is the biggest change for this example and therefore likely to have somewhat of a "glass jaw" compared to the original creature - only 28 maximum compared to 189, which is likely to be the case for many similar reanimated corpses, particularly as this is calculated from the feralgeist's *current* hit points.

As Spirit Bind does not impart the knowledge of the original host creature, the possessing spirit may not use the troll's Ice Breath as consistently and efficiently for example when it originally reanimates the corpse, but presumably gains some familiarity with its host corpse's capabilities over time.

My interpretation is that the feralgeist using a possessed corpse will try and adapt to the combat situation, but attacks without thinking, isn't particularly good at choosing targets, and fights using the corpse as a disposable weapon until the reanimated body is rendered completely inert (0 hit points) rather than the preserving it as it can fly off through obstacles to escape or wait out the recharge. This compares to the living troll, which is more instinctive and less likely to change approach but more likely to avoid combat, can parley to those that speak Molgur-Trul, and will flee if moderately wounded (30%, 57 hit points). 

Other Feralgeist Reanimated Corpse Variations

Note that technically RAW, Spirit Bind above allows the possession of the corpse of any *non-humanoid* creature - amongst the obvious options such as beasts (a dracodile anyone, shhh don't tell my players LOL...), dragons (the various drakes), giants (hence the winter troll as an example), and the rare monstrosities, this also includes aberrations, constructs (steamjacks etc), as well as swarms (eg a Scylla flock) and technically other corporeal undead. 

I don't think it's intended to include celestials, fiends, or fey - my observation is that the broader implications of 5E mechanics are not as robustly considered in the IK:R material to date.

This makes for multiple interesting variations which I don't have the time to go into here.

Wyldgeists?

I can't find an earlier 3.5E version of the feralgeist, but a very similar creature referred to as a "wyldgeist" exists (Monsternomicon Vol 1, pages 206-207) with a "spirit bond" power allowing it to reanimate dead corpses, *including* humanoids which is a notable difference. They also are acidic in their natural form, discouraging attacks.

 A Pendrake Encounter featuring wyldgeists can be found in No Quarter #28 (pages 68-71) detailing a scenario where the wyldgeists reanimate bandits that attack a caravan.