Some general feedback —
- "each containing one unit combined with a hero" — is the intent here that each army contains only that one unit? If so, that could be explained more clearly; I only realized this might be the intent upon reading the rules for the Abomination (up until then I thought this either meant only one of your units may have a hero attached, or at least one unit must have a hero attached).
- If the above is true, then I'm worried that players simply taking turns activating their one unit might easily devolve into a somewhat boring case of "rolling attacks until something dies". Which, by the way, might happen very quickly — whoever first fights the Abomination in melee is likely going to get like 50 attacks in their face which is at least gonna put them at risk of routing, if not an immediate wipe-out. Since most models it kills are probably going to get added to it, this would almost certainly lead to an exponential power increase that will make it effectively impossible to destroy the Abomination.
Also some minor notes —
- "All the players must collectively decide on a single starting location. And must be deployed within 6 inches of each other" — how large an area should this "starting location" be? Also, if all players share a deployment zone, the note that they must deploy within 6 inches of each other seems redundant (as well as somewhat unspecific).
- For the core of the abomination, I think it may have been more elegant to define the unit's stats in the mission itself instead of referring to an existing unit from an army book. Getting to pick a unit also means a lot of possible variance with the challenge: a Tough(6) Bat Beast or a Tough(12) Giant God-Statue will come with very different difficulties, and some such units might come with special rules such as Caster that the mission doesn't tell you how to handle.
- The Part of the Core rule doesn't account for the fact that the unit might have Tough(3); should probably trigger when a model would be removed, rather than when taking a wound.