When I saw Extermination, I knew immediately that I wanted to use Dynat in an Alpha Legion army list -- pre or post Heresy.
The main thing that attracts me to Armillus Dynat are his special rules. Firstly he can deep strike an infantry unit in to play. Secondly, he provides a bonus to damage against vehicles inside the enemy deployment zone and permits re-rolling of sweeping advances. These two can combine together very nicely. And that's before we even consider than the Alpha Legion could all potentially infiltrate in to position in the enemy deployment zone. The only real question to build an army list around Dynat is what Rite of War to take. I can see a number of possibilities here, but I'm going to aim for the Alpha Legion rite of war Coils of the Hydra. Equally, I'm not using it to take any units I couldn't already. I'm doing so just because I can. To mess with the opponent. Because I can. Or perhaps to leave the door open for units like the Raven Guard's Mor Deythan. They would really be good here, as would a close combat squad.
With the above in mind, I need to take 3 infantry squads. This really means less points are available for the nice stuff at this level.
But the real challenge is thinking about what I want to deep strike and why. To combine with Dynat's special rules I want either something that's going to sweep in combat or something that's going to hurt tanks. Hence I opted for a melta gun support squad (I play against 40k lists, so armoured ceramite isn't such an issue), or terminators in tartaros armour (for both purposes).
On top of this, I wanted something that would be slightly unusual to play against. So I actually thought about taking some lascannon tarantulas. I'm still not sure about that, but it seems very Alpha Legion (setting up in advance and all). In the end, I opted for an all infiltrating army under the mutable tactics from the Alpha Legion -- that's unusual enough at least. Here's the list:
*Armillus Dynat, power dagger (205 points)
*Master of Signals, melta bombs, power dagger (95 points)
* Tactical squad with 20 members, vexilla, sergeant with power fist and artificer armour (285 points)
* Tactical squad with 10 members, combat blades, vexilla, sergeant with power fist and artificer armour (205 points)
* Assault squad with 10 members, 2 plasma pistols, sergeant with thunder hammer, plasma pistol, melta bombs and artificer armour (330 points)
* Tactical support squad with 5 members, melta guns, sergeant with power fist, artificer armour and melta bombs (190 points)
* Recon squad with sniper rifles (150 points)
* Terminator squad in Tartaros armour, 1 plasma blaster, 1 thunder hammer, 2 chain fists (220 points)
* Heavy support squad with missile launchers, augury scanner (165 points)
Plus a few points left over to scatter some power daggers here and there.
Deep strike in to position either the melta guns or the terminators for some early turn fun. The rest of the army can infiltrate in to position once the enemy is set up. That's the point of running an infantry heavy list. Depending on the enemy, it might be better to get one of the tactical squads deep striking in, although fundamentally, they're there for objective taking.
The back field is made up of both the recon squad and the missile launchers aided by the master of signals. Realistically, they're not back field, but more infiltrated in to the enemy deployment zone hopefully. And the master of signals is there for BS=5. Amillus can provide similar effects, but I think I'd want him hiding in the 20 man tactical squad most of the time.
The assault squad meanwhile provides rapid moving pressure to ensure early assaults and key units are tied up. Overall, I'm confident of seeing off tanks in the early turns of the game and mopping up key elements by grabbing first turn and doing some careful positioning to pull off an alpha strike (pun not intended).
To modify it, I might take Mor Deythan in place of the recon squad, or be tempted to increase the size of the terminator squad. I've avoided vehicles, but could be tempted by Sicarans, Storm Eagles, Death storm drop pods (seriously!), contemptors and other assorted nastiness. Chaplains, Apothecaries, and librarians actually tempt me too here. Automatically exploding tanks is possible too with the Alpha Legion consul, but only if playing relic related narrative campaigns or missions. In any regard, there's a whole load of power armour on the board here, and that's a real under-stated strength of this list.
Overall, a fluffy and effective list that still gives opponents a chance I think. Comments welcome.