Heres something Seattle GW bunker did a while ago. I remember
it was quite fun.

Set Up:

2x2 Table, Divided up in fractions of a 1 foot lowered circle. 8 sections
in the corners, Each player has a home base on located on the outside of
the circle.

2D6 Randomly generated asteroids, you can throw, push and move around
with scatter dice. STR 8 Hit. 1D6 And one Scatter dice is used to determine
direction of asteroid being pushed. Models must make base contact to be considered thrown.

Optional: A Token may be placed in the middle of the arena for the purposes
of activating and de-activating a player. The player who reaches the token may
decide to whom to de-activate. Another player must activate the de-activate player.

RULES:

1 Dreadnaught from; Space Marines, Chaos, Eldar (Walkers) ,
I.G. Sentinel Squad, or Tau Crisis/Broadside Squad is chosen. I dunno
if Dark Eldar have any, but Necrons may use destroyers as a squad.
Tyranids may use 1 carnifex or a Tyrant guard squad. Ork Cans and
Talos dreadnaughts may also be selected.

Follow codex for upgrades. Alliances may be made/broken.

TURNS:

Players roll off to see who goes first, 2d6 are used, if similiar rolls are made
take the two, or however many players that rolled and roll off, Repeat process
untill turns are assigned.
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OK now that mumbo jumbo is done, any contestants?

What, no Ork dreadies?

Dark Eldar should be allowed the Talos.

And I don't see why an ork dreadnaught wouldn't be allowed, it is a dreadnaught, more so than destroyers or a senitinel squadron.

Anything D.E. Would have elsewise besides Talos?

guess not...they only have the talos.

My local hobby store ran this same event once and, surprisingly, one of the most effective models was the Tyranid Carnifex. My friend brought one and won several of his four way matches with it.

I won the tournament overall with three imperial guard sentinels loaded up with lascannons and HK missiles. Its amazing what an HK missile can do for you in a dreadnought arena battle. ;)

Yeah I'll say, I have a HK missile on my B.A. Dreadnaught
Plamsa Gun and a storm bolter with a power fist, it was pretty
effective

You forgot the Penitent Engines for the Witch Hunters. ;)

Back in the day we didnt have penitient engines.

Hmm think a defiler would be allowed? As a substitute for the chaod dread?

I dunno if it would be balanced tho...chaos does have its dreadnaughts.