I tried to be concise but there's probably some nuances that I'm overlooking from being so familiar with the lists.

Expendable: I added clarification here (incorporated some FAQs into the rule andexplained how it interacts with crossfire and disrupt) and made it a generic rule. Game-wise it plays the same.

Tunneler: Again, additional clarification from the main rulebook (when you do things, how you place Tunnelers, etc). It plays as it does from the rule book, except all tunnelers have to be within 15cm of the surface point (there was some clamoring for this on the forum).

Resolute: I added this but nothing uses it yet. The plan was to give to Trygonsand Harridans as I didn't want to make them fearless.

Planetary Assault: Is completely new. It's meant to represent the “rain of spores” instead of planetfall.

The Hive Mind: This replaces Relentless. Instead of +2 to engage and rally, it'signore the -2 mod for being broken when rallying.

Mobility: Won't be around in the next interaction. I figure we could cut it out and just give things Walker.

Synapse and Brood: I made these their own rules for clarification. There were used in 9.2.1 but never defined separately from everything else. I tied Expendable to Brood, game-wise it's the same but clearer I think.

Spawning: This differs from 9.2.1 in two notable ways. One, units can only be re-spawned to their parent formation. This came from Jaldon and after the tests I think it helps with the balance (and allows us to go to a regular BTS). In 9.2.1 you had one big spawn pool and could bring things back where ever you wanted them (assuming you rolled high enough on your spawn roll). You can't do that in Onachus. If a swarm has 4 Termagants and 2 Hormgaunts then it can't spawn back 5 Termagants, or a Carnifex, etc. Two, 9.2.1 had spawning as 2d3 and then you had -1d3 for certaincriteria. I thought that was confusing (“do you roll 2d3, and thenroll a 1d3 to subtract?”). To clear that I up, I just went all positive: if you're not broken, you get 1d3; if you're not within 30cmof the enemy you get 1d3.

Tyranid Swarms: This was in 9.2.1 and I dropped it. It pretty much enforced Synapse Range (all brood with 15cm of a synapse create) and allowed Brood to be moved from formation to formation. The former didn't really seem to add much to the game and just made for one more thing to remember. The latter was removed so we could go to a regular BTS (more on that below). It's still kind of around with absorb from Synapse Swarms (see below) but in a very limited manner.

Tournament Scenario Victory Conditions: This was replaced with Synapse Swarms in Onachus (sort of a catch all army list rule). In 9.2.1 Brood couldn't contest/control objectives. They can now in Onachus, but only if their Synapse Creatures are alive. In 9.2.1 BTS used to be you had to kill half or more of the Synapseunits on the table. I thought this was pretty radical. It's the only special rule I know of that changes a victory condition and I don't think that something that core to Epic should be tinkered with. Now BTS for 'nids is just BTS. Onachus is able to do this because you can only spawn back swarm losses and don't have reorganization. In 9.2.1 Tie breaker had its own calculations. Again, I didn't want to mess with victory conditions so this was dropped.

Synapse Swarms: Like I said above, this replaced the Victory Conditions rule and tidied up a few other things. I handled what happened to brood units when all of a Synapse Swarms' synapse units were destroyed. This was a confusing point in 9.2.1 and I'm still not sure how it's supposed to be handled. What I did in Onachus was say that they go to initiave 3+, can't capture and contest objectives and count as destroyed for tie break. So, opponents are rewarded for “shooting the big ones” (the formation counts as destroyed for tie break) and Tyranids revert to their animistic tendencies when they're not controlled (they have a hard time activating and can't do anything with objectives). The latter is how I represented Synapse Range in the list.

The Synapse Swarms rule also allows you to absorb these leaderless brood creatures. In 40k, brood out of Synapse Range retreat towards the closest synapse creature. Rather than try to write a rule to that effect I just made it worth the player's while to do it. Absorbing is pretty much the reorganization part of the 9.2.1 Tyranid Swarms special rule. The notable difference being that an absorbed formation is counted as destroyed for the purposes of Victory Conditions. This is so we can have a regular BTS. If the BTS's synapse is gone, it counts as destroyed for tie break, but you still have to kill all the brood that's left for BTS. To keep the Tyranid player from interfering with this, if they absorb the BTS into another formation the opponent gets BTS.

Army List: The differences in the army list are pretty clear. I moved to the Ork model. In 9.2.1 you bought Synapse (which couldn't total over 1/3 ofthe list) and added Brood to it. And Independent Swarm couldn't be more than 1/3 of the list as well. Rather than putting the onus on the player to check to see if their list is legal I figured a list that enforces itself would be simpler. The only restriction is the normal 1/3 for titans one.

½ Gaunts Rule: This rule upset Matt and Chris to no end and I dropped it. It made Hormagaunts and Termagants lost in an assault only count as ½ of a unit. So if you lost 5 of them the opponent would only get +3 to resolution. When I played with this with 9.2.1 I found I didn't need to prep an assault and lay a BM or even worry about support too much. I just piled the gaunts in there and went for it. It just felt like a tremendous crutch and as with BTS and the tie break, I didn't want to tinker with a core mechanic of Epic so I dropped it.

Initiative: Onachus is 1+ (and 3+ for synapse-less synapse swarms) and ignores the-2 for being broken to rally. 9.2.1 is 2+ and gets a +2 to rally and engage.

Mobility: Won't be around in the next interaction. I figure we could cut it out and just give things Walker.