For those among us still playing 3rd edition; I have added the stats for the Capitol Imperialis (originally by Tommy Royu) to the "Epic 40,000 Armies II" book. You can find the PDF at my homerules website.

Good proxies would be:
- Hawk Games Ferrum Class Drone Base
- Onslaught Miniatures' Grud Tartarus
- ...*

(* Add more below please.)

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Epic 40,000: Exodites, Necrons, Dark Eldar, Sisters of Battle, Tau

BLOG: Epic 40,000
BLOG: Warmaster

Toco's Mod: Allied HeroQuest

When you are in need of knowledge, check this out: http://miniwars.co.uk/epic/units

Edit: actually, only after posting, I realized that only what appear to be NetEA units are represented there. Pity that. Well you could always add that what's-it's-name from Troublemaker. Nice model.

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Nitpicks Knapsack (Scratchbuilding and 3D designing and printing and painting)

Troublemaker has Leviathan sized models. But also Imperialis sized?

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Epic 40,000: Exodites, Necrons, Dark Eldar, Sisters of Battle, Tau

BLOG: Epic 40,000
BLOG: Warmaster

Toco's Mod: Allied HeroQuest

True. Even though the Troublemaker machine is a lot bigger than the Leviathan, it is probably still to small to pass as an Imperialis. It looks much bigger. Sorry for the confusion.

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Nitpicks Knapsack (Scratchbuilding and 3D designing and printing and painting)