This 1 pt. solo has quite limited usage. While he can sqeeze into a lot of lists, he's not often seen on the tabletop. Why? Well his signature ability is that he can basically possess a living warbeast (friend or foe) when it dies. The warbeast heals a point in each aspect and becomes functional again! The downside? The warbeast no longer counts as part of the battlegroup and can no longer be forced. This normally is a huge disadvantage with heavy warbeasts. Most heavy warbeasts are MAT 6, at best, and often rely on various buffs and/or the ability to force to get them to appropriate levels of efficiency. While a feralgeist-possessed warbeast can still receive some buffs, the beast possessed no longer is part of the battlegroup and still can't be forced, and when most heavies are sporting P+S 16, at most, the feralgeist isn't "OMG BROKEN." That is, until we consider the Mountain King.
As far as I know, there's no reason why a feralgeist cannot possess a Gargantuan. They count as warbeasts and as long as the beast is living (sorry, Woldwrath), it can possess it when it dies! Sure, it loses its animus and can no longer be forced, but does that really matter when you have a massive beat stick? With these monsters being able to absorb a lot of damage, having one of these cheap little solos around makes the entire prospect of killing one all the more daunting for your opponent. You might think that not being able to force is a terrible thing, but the Mountain King can totally still wreck stuff since it's P+S 19! Granted, Trolls have a lot of faction-specific spells and buffs that the Mountain King would no longer be able to benefit from, but you'd either get an extra turn with your gargantuan or your opponent will need to expend more resources to kill it for good.
We haven't seen the other Gargantuan statlines (except the Woldwrath but he has Spell Ward) so for all we know, the others might also have MAT 5 and the Mountain King isn't as bad as everyone is claiming. What we do know is that Trolls have access to a lot of knock-down effects (as I highlighted in my original article). A low MAT is irrelevant when you're automatically hitting in melee, and then you don't have to worry about not being able to force to boost attack rolls.
The funniest part? Although the Mountain King can't be forced and is no longer part of your battlegroup, it can still use whelps to heal itself. If your opponent does manage to kill the MK and the feralgeist possesses it, and your opponent can't finish him off, suddenly you can heal with all of those whelps you've shed and suddenly you can have a Gargantuan that, while still severely limited, can still deal some damage (especially since it can still trigger Kill Shot).
All of this for a lowly 1 point solo. It might seem like a silly gimmick, but for 1 point you don't need to have very high expectations. In fact, you can merely consider it an insurance policy that will make your opponent think more carefully about charging the Mountain King. With such a high native P+S on his attacks, not being able to force is not a terrible thing - especially against heavies (his prime targets anyway). Make sure you have ways of knocking down models and you're golden! Here's a list to chew on:
pMadrak (+6)
- Bouncer (5)
- Mountain King (20)
Kriel Warriors (full, 6)
- 2x Caber Throwers (2)
Kriel Stone (min, 2)
- Stone Scribe Elder (1)
Fell Caller (3)
Feralgeist (1)