The **Pariah Nexus** Crusade book for 10th edition **Warhammer 40,000** primarily tells a story about the Adeptus Mechanicus and Necrons.
While there is a small provision for "interloper" factions on page 101, it doesn't offer much motivation for a third party army to join in on the battle.
Here's my d6 table with some specific motivations to help guide your army through the Nephilim Sector:

1. You support the Necron cause for reasons known only to your mystics.
2. You were just about to attack the planet when you noticed it was an active warzone, so you decided to join in the plunder.
3. Everyone else is getting Blackstone, and you'd like some too.
4. You detected fighting in this direction and have come to assist.
5. You want to help end this war so it doesn't spread to your own sector.
6. You love killing anything that moves.
