The mutalith can be summed up in a few words; random, cheap, bait, behemoth. It fulfils all of these functions and is surprisingly survivable as long as you control the movement and control the damage done to it.
It is a very large and impressive, perhaps intimidating, model that can do a lot of damage. It is also not as effective as it looks due to being almost entirely random. Stick it on a flank and go after the largest horde on the flank and hope for the best!
Units That Synergise With It
Nothing really. If allied to StD then the sorceror’s re-roll all 1’s can help, as well as the usual obvious realmgate spells, but largely it’s a solo beastie.
How to Use the Unit in Game
Stick it on the far side of a flank and go for those objectives. If you put it in the middle it may get some extra chances to roll a 6 with it’s ability, but it almost certainly will die within a few turns if you do so.
Pros & Cons
It’s sort-of good against everything, until it charges into combat it is pretty good with its odd abilities against hordes, as it can apply pretty effective de-buffs. If you roll well it can be worth as much as a magma dragon, if you roll badly it can be worth as much as a pointy stick with the point dropped off.
It takes ages because it is such a fantastic model. The Cthulu like tentacles, and the sheer bulk of the thing make it extremely fun to put together, as it is a spawn variant it is really easy to convert, and it just deserves the time spending on it. If you like modelling and painting at all then this is the model for you. It is one of the older plastics so please do be prepared to spend some time trimming the mouldlines!