In anything 1600 up I will run two Terrorgheists, Unless of course I am fighting a Cannon heavy(or long ranged shooting) army in which case the 450 points is then spend on Hex Wraiths or Cairn Wraiths.
I usually keep them close enough to the general to get a march move and behind my buses & tarpits. This allows me to march fly them over the top of units that are charging my tarpit and spin them at the end, this gives me one scream, followed by supporting rear charge and a second scream and thunderstomp etc, etc. All within the safety of my IoN bubble.
Of course this means you are risking a rear charge from an enemy unit into your glass cannon BUT if you can win combat and cause the enemy to flee OR raise a new unit of zombies behind the Terrorgheist then you are safe(ish).
Then towards the end of the battle I have my Terrorgheist chase down and circle units screaming them to death in a sort of duet, always jumping out of their charge arc.
I of course cannot stress it enough if the cannons out number your Terrorgheists don't take them, If the cannons are likely to outnumber them don't take them. Replace it with something more numerous Grave Guard, Black Knights that sort of thing (My preference is ethereal).
Dwarves are different since their magic rune cannons can hurt Ethereal but there is nothing quite as sweet as the look on an opponents face who expected you to take large things and has sunk points into non magic cannons or large units of non magic range attacks and you have instead turned up with 3 little squads of Ethereal Vanguard units that cause Terror.
I can understand the desire to take a Mortis Engine and a Terrorgheist, I have run this a few times with little success. The following problems arose:
- For the Terrorgheist to get its +1 to regeneration it has to be close to the engine which grounds it from making a 28" scream, and really why else do you have it? The thing is awful in ranked CC as it dies to combat resolution and fast large volumes of low strength attacks. It also hates impact hits. (My Terrorgheist has been the bonnet mascot of tomb king chariots one too many times for my liking)
- If one of them is lost then your combo is down the drain and you are left with one or the other (which is not a bad thing by all means) the other is then likely to get picked off quickly enough since the enemy can now focus fire.
- If the Terrorgheist is lost then you can move the Mortis engine to try and give the rest of your army a +1 to regenerate so this is the preferred option. If you however lose the Mortis Engine then you keeping the Terrorgheist close to it has probably (unless we are talking about turn one cannon fire [I warned you about cannons bro]) then you have probably lost some time and wasted its manoeuvrability/ Fear factor.
So to summaries:
- double Mortis engines in a horde army, YEAH ! (200 5+ Regeneration zombies for the win ladies and gentle men)
- double Terrorgheist Vs a CC or short range shooting army, YEAH BOI!
- One of one and another of the other, Id give it a miss honestly.
TL : DR
2 Terrorgheists all the way.
~Aterak