Denied The Use Of a Mod for my Mod

[Ludens]

What I want to know is, can he actually take my mod down with the EULA? Can I still make my mod and just say screw him and upload it anyway?

Hello Tom Selleck's Mustache, welcome to the .Org

To answer your question (and with the caveat that I am not a lawyer): I am afraid he is right. Modifications do count as intellectual property (IP), so you are not allowed to distribute them without permission of the IP owner. This applies even if the modification is based on someone else's IP (in this case: Rome: Total War), and whether or not that person has given permission. There are exceptions to this (if the mod was explicitly released as public domain, or under certain Creative Commons licenses), but you have to check if these apply.

You can, however, modify his work for private use: that's not an IP violation as far as I know. You just cannot release that to others (and uploading it would count as a release).

I am sorry to have to be bearer of bad news.