Here you have decided in the 8th edition of Warhammer 40 000 to play Apocalypse. You open the Rule book to see the rules, and ... There's nothing there. Well, then you open the Chapter Approved, because there they promised special rules for all game modes, and ... there you can see three missions, a ton of information on how best to organize the event itself, and some general advice. And that's all. No assets, features, formations, and so on. How are you supposed to play it?
It is quite logical that you can compensate the absence of rules only by yourself, inventing your own. The 6th edition Apocalypse rulebook night help you with this - you will not find anywhere else as much diversity as there. It is quite logical that it’s better to adapt the old special rules to the realities of the new edition, instead of coming up with completely new ones. Some even can work without changes - like, for example, an asset with a smoke screen.
Sounds like a lot of work. But, since you decided to organize Apocalypse - you should have been ready for this. Find out in advance who is planning to come, and what kind of roster they have. The balance on this format is usually very unobvious, but try to determine which side is stronger, based on units, not their point value. If one side has 10 Baneblades, and the other doesn’t have a single LOW, it is unlikely that the points will mean anything.
If you see the obvious superiority of one side - try to balance it. For example, forcing to leave a part of the units in reserve, or giving the second side additional command points. You can also forbid the players of the strongest side to discuss each other's tactics during the turn itself - a similar rule exists in Age of Sigmar and works fine there.
The concept of command points is great for the Apocalypse, and you can easily assign prices for each asset. Only, given the scale of the game, it will be better if the players will have the opportunity to restore these points - for holding special objectives. It will be better even if some objectives will give command points, and the others – victory points. Then the players will have to choose, which is always interesting. Plus - you can generate additional command points for the presence of certain characters or units in the army – this way you will stimulate players to create thematic armies.
GW is not in vain paying much attention to the organizational part - in addition to the special rules, a successful event implies a competent and clear organization. Since this is definitely not fast, you can use incentives to speed up players - for example, to track the time for the deployment. The side who made it faster - goes first. If you don’t have a couple of days to play calmly and leisurely - use a time limit. Maybe the planet on which you are fighting is being destroyed right now? In this case, you can also use one more type of points – the ones giving additional time for the next turn.
In general, the more additional rules and conditions - the more interesting is the gamegame. So, do not be afraid to invent something original, or use somebody else’s work. The balance at suc scale becomes less sensitive, so a bad rule is unlikely to destroy the game. The main thing is to arrange in advance with the players, whether they need such innovations, and decide which ones you will describe before the game, and which ones will become a surprise. Lists of Assets and Stratagems definitely should be offered in advance, even before the stage of rosters’ building. In the absence of official rules, all that remains is to invent your own. So, use your imagination and go forward!