How on this green earth do I make Atropus for Spelljammer? I get that it's a Pathfinder/older DND abomination but my gods do I want to put that in this game soooo much, as well as the Angels he comes with. I don't want it to be an impossible fight but I don't want it to be easy, this would be my first time making such a big boss for a campaign.

I had to go away and look this up.

It looks like this inst just a boss you put in, this is the entire aspect of evil for your campaign. The is the super big bad, a god even. The Aspect of Atropus is a CR23 so even that is super powerful.

This is a campaign ending fight. Put it in as it is and if players die they die. This is the way the storie goes.

The players should never fight it head-on. Atropus is a sort of lovecraftian horror, a being so far beyond humanity that we are to it what ants are to us. No mortals, no matter how strong, could ever hope to scratch it. Instead, I would say that “fighting” it involves achieving a number of goals to mitigate its effects and drive it away. It’s said that Atropus is attracted by death, so perhaps some burst of be romantic energy elsewhere in the universe could lure it away. Similarly, the radiance of positive energy could drive it off. Or perhaps there is a particular necromancer that is summoning it here, and it will leave if they are destroyed or stopped. At some point, I’d suggest making the players need to actually land on the dead planet itself, to extract some item or establish some connection.

This helps me out so much more than you two know. Thank you both! Maybe I'll have them just face the Aspect to try and not do a TPK, but if they want to continue and get past the CR for the Aspect, maybe in the future I can get them to where they can eliminate the god itself.