I think the only examples we have about writing modifiers is the document linked above and again here unfortunately. The smooth the path example shows how it works but if you’re still having troubles getting your exact idea down I can walk you through it 
Give it a shot and get back to us if you get stuck.
I’m currently using recast graph, but I’m using it with a tile size of about 128. Is it no different from grid graph if I reduce the tile size to 1?
I thought maybe I could achieve what I wanted if I cut it down to 1 and give the penalty tag where the zombies are.
As far as I know, RichAI and Custom Modifier can’t be used together, so how can I do it?
The docs seem to suggest that custom modifiers may be able to, but information on it is a little spotty. To be honest, I think this may be a reason to explore one of the other movement scripts. The list of all movement scripts explicitly says this about RichAI:
- Kept for compatibility only. If you are starting a new project, the FollowerEntity script is better in almost every way.
As stated before, you don’t need to know ECS at all for FollowerEntity, and even if you have another reason for that, you may want to try AIPath.
From what I’m reading it looks like the easiest way to conceptualize it is that Grid Graphs are better for small dense worlds while Recast Graphs are better for large sparse worlds. It’s more nuanced than that but that’s looking like a good rule of thumb.
You could always give it a try though and if it doesn’t work try something else. It just may work